That is sort of like asking "what's the difference between home movies and Hollywood films".

The maps here are made by people as a hobby (I am one of them). The best of them will have some special feature that makes them stand out. Then there are many which have just been put together by formula from the same sources, using the tutorials on this site. Nothing wrong with that of course. Many of these are excellent and will be all that many people need.
To dig a little deeper, Garmin considers their map format proprietary and they have never officially published the specifics. They have special software to create their maps and they guard it very closely. It is not available to people outside their own network. So all the free maps here and elsewhere were created with reverse-engineered tools. These tools are far behind Garmin's proprietary tools. The author of the software used for most of the maps here ceased development of his program a number of years ago.
Garmin topo maps have elevation data (DEM) that will generate shaded 3d views and no third party maps can do this. Garmin's 24k topo maps also have routable roads that can give you turn by turn directions. It's possible for a third party map to provide this, but only a few of the thousands of maps here can do this because it invloves a lot of work.
Garmin 24k topo maps also have a rich set of POI's - things like restaurants, gas stations, stores, etc. They have to resources to purchase this data and the time to compile it into the map. None of the maps here have that kind of detail.
But the good thing is, the maps here are FREE. So what do you have to lose? Just download whatever looks interesting and give it a try. If you like it, don't look back and just get out and enjoy yourself!
If you do purchase a Garmin map, get the DVD version if possible. Garmin seems to be discontinuing these. But there are some big differences. They are more expensive, however if you actually compare them with the download and memory card versions, you will see the DVD version covers more states. But the big difference is, there are no restrictions on using the DVD maps, so you can install on all the GPS devices you own. You can also make backup copies to protect you investment.
The preloaded cards can be used in any gps, but they cannot be duplicated. They will only work on the original card, so if anything happens to that, you're just out of luck.
The download version give a little more safety because you can download them again if needed for 12 months following purchase. You can also make backup copies and they are not tied to single data card. However, they will be permanently tied to the gps they were purchased for. So if you decide to sell your GPS in a few years, you would not be able to install this map on your new GPS.