I had a Garmin 100 and traded it in on a 300. I really like it. I will preface it with I've not had another brand. The things I feel have been great. I don't have to have a beeper in the field to find my dog hence I don't feel like I'm hunting with a dump truck. The Garmin 300 is really a gps with dog training abilities.
I like if someone I'm hunting with has a Garmin I can link or units and I can see where they and their dogs are. Plus if you link your units together if you lose your handheld your busy can track it down for you. A buddy put his handheld on a bedrail of the truck while putting his dog away and drove off with it there. Needless to say it wasn't there. After a frantic search of every coat. I pulled it My handheld we backtracked a mile down the road and found it. Plus if your linked and train with someone he plant birds lay tracks or what ever and watch the them in the handheld in real time, see where the track or birds are. I also have alerts when the dog gets past a self picked radius from me and a point indicator.
You can also put onx SD cards in it. It has also helped me be a better dog handler in that knowing where your dog is, is huge, even on windy days. The dogs chase a deer, you can still find them. I would recommend looking at the capabilities of each unit and see if other capabilities and imagine what be of help to you. They really are more useful than being a dog locator.