I guess nobody else is interested in this, but I'll update this thread anyway… maybe it will help someone in the future. Here's what I've learned so far about making maps on my iPhone 6s Plus
Jot Dash StylusBare fingers are not the ideal tool for precise drawing on a small screen, so I got a Jot Dash on Amazon.
http://www.adonit.net/jot/dash/It works great! I don't quite understand how it works, but it has an internal battery and USB charger. If the power button is off, the stylus doesn't work. But it isn't bluetooth or a radio signal. Apparently there's some kind of electrical charge at the tip that allows it to be recognized.
iGISFor a free app, this is really very cool. No problems importing/exporting shapefiles. And I found a way around using the MapTiler software to create compatible raster imagery. In GlobalMapper I'm using "Export Web Format…" and choosing the "OSM Tiles" option. After the export finishes, I just change the name of the HTML file to "googlemaps.html", which is the name of the file that MapTiler creates". Then just zip the whole folder and drag it to iGIS in iTunes.
This opens up fine and does not seem to have the problems I was experiencing with MapTiler. If the raster image is too large, that does cause a problem though. I had issues with freezing and incomplete screen updates with a 500MB raster image that consisted of about 8,000 tiles. A 170MB image with 500 tiles works fine though.
I did the $5 in-app purchase to upgrade for one month of the iGIS pro subscription. I don't know, it doesn't seem to add a whole lot of value. For $5 (or $20/year) you get a measuring tool, the ability to have labels display, a few different line/point/polygon styles, a map scale bar, search and display of GPS accuracy. The different feature styles are not very attractive and can only be applied to whole vector layers, not individual objects. The only one of these features that I find especially useful is the map scale.
So, considering that the basic app is free, it really raises the bar for what you should expect from the other expensive apps.
CartoMobileThis app costs $80 and has not been updated since 2014. I was concerned as to how well it would work under iOS 9 and the 6s+. So I wrote the developer and immediately got a helpdesk autoresponse with a ticket number. A couple hours later I got a nice response from one of the developers who said they were definitely still supporting the app and were working on an update. He also invited me to join the beta test group. So within a few hours of contacting them, I had a (free!) beta version to play with. Great service, very impressed!

There are a few issues regarding how things scale to the very high resolution screen on the 6s+, but they're more of an annoyance than a "problem". I found it very stable with few crashes. The really nice feature of CartoMobile is the ability to directly use GeoTIFF files without turning them into tilesets. I tried some pretty big ones, around 700MB I think, and they worked but were sluggish. Smaller images worked better. Overall I'd say the interface is a little smoother than iGIS when dragging around a raster image.
Shapefile import/export is fine. You can only have a single style for each vector layer however. I think iGIS is superior in this regard, as you can store hex color values in a shape attribute, then use that attribute to control the color on a feature-by-feature level. I wish CartoMobile allowed this.
This will also be a very nice app once they get the bugs ironed out. But possibly a little hard to justify the $80 price just for the ability to import geoTIFF when iGIS is free.
GIS Pro/GIS KitThese apps have a much richer feature set. There are some nice videos on Youtube that showcase this. For example
https://youtu.be/P7hBfWZGAIUBut looking at the reviews on the app store, they were really positive until 2014, at which point people said it was a great app but crashed all the time on iOS 8. The only new review, from Oct 2015, was very critical. That's not good when the basic version is $100 and the pro version is $300.
I wrote the developer and got a response within 24 hours. They addressed the iOS 8 issues with a couple updates last summer but it has not been tested on iOS 9 so he couldn't offer any insights into how stable it would be. He conceded that it was badly in need of an update but was frank in saying their development team is small and overworked, so I shouldn't expect an update anytime soon. I asked what recourse I'd have if I bought it and found it unusable. He said they would do their best to help resolve any issues, and I could request a refund from Apple if I wasn't satisfied.
So I'm going to think about this a bit. It really has a lot of nice features not available in the other apps.
Map EditorThis is a curious one. Only $2 so I figured it was worth a try. The developer claims
"#1 Top Paid iPad Navigation App in Japan and several other countries."
http://www.o2gis.com/vi/ogeditore.jspHave only tried it once and still not completely sure how everything works. Some functions bring up dialog boxes in Japanese. But it has a few powerful features - you can modify the geometry of vector objects and move them around. These seem like basic functions of any GIS app, however neither iGIS nor CartoMobile can do this (GIS Pro can though).
I created a few features and exported them as a .kml file. It opened up in Globalmapper with no problem. Not sure how much I'll use this, but it's kind of an interesting app.
Garmin GLOI got this a couple years ago and it works great. Absolutely no problems pairing and using it with the phone. I am just disappointed that you have to "take it on faith" that the GLO is actually providing data since iOS gives you no clue aside from showing it paired in the bluetooth menu. There's no way to see a satellite screen or any other data directly from the GLO. I got a couple free apps - GPS Status and GPS Data that show a few things, but they're limited to the scant info that iOS provides them.
Recording tracks in the Galileo app, the tracklog shows about 6 position updates per second (the GLO can provide up to 10/sec). I got a belt clip for the GLO and have been attaching it to my hat for an unobstructed signal while walking around. Also rigged a pole I can clip it to and plant in the ground for collecting static points.
iPhone 6s PlusReally happy with this phone as a handheld GPS replacement. Am using an Otterbox Defender case that comes with a holster/belt clip. Dropped the phone in the mud in the swamp once, so the case is earning its keep.

When you put this big phone in that case, it definitely says "tablet", but I actually like that. For "everyday" use, I pop the phone out of the Otterbox (which is pretty quick once you get the hang of it).
With all the different GPS and GIS apps, it opens up lots of possibilities that aren't available on a Garmin handheld. The screen is beautiful and battery life seems good. I was out continuously running the screen at high brightness and receiving data from the GLO for about 3 hours. Didn't even use half the battery for that.