For someone that is just getting into the use of a GPS, with any of the more sensitive and latest technology GPSs to become fairly proficent with it will require quite a bit of hands on practice and experimentation and be willing to ask a lot of questions and reading some very good wiki links (How To Instructions) for some models.
Once you become comfortable with it you'll never leave home without it.
Since many horse camps / trails have paper maps of riding areas. I would think that the ability to put these paper maps on your GPS to navigate by would be very benenficial, but another learning curve envolved. Also satellite aerial imagery would be nice.
With all of the above in mind the Garmin units that will do all of the above are the Oregons, Dakotas, Colorados, 62 and 78 series units. The touch screens are probably a litttle easier for a complete newbie to learn, so this brings the list down to the Oregons and Dakotas. The Oregons have a larger screen. For around $330 (on sale occasionally for $250) you can get a very good Oregon 450 and then you can put a lot of free maps on it from this website. You would probably be well pleased with the 450 for a long time.
A lot of the final decision comes down to personal preference and the intended use. The best thing to do would be to look at them in a sporting goods store with knowledgeable sales people.