Try them both!
I started on the 300C (it was all that was available in the area). I got to the point of soloing (~9 hrs) and after getting half an hour solo time in the 300, I had to go back to school where Robbies were the only option. I've been flying R-22s since and enjoy them much more than 300s. Cruise in a 300 is slow in my opinion, and feels strange because you're staring at the ground with your neck craned up to look forward. The stick forces on the Robbie are lighter, partially negating the need for any trim anyway. The Robbie is tight for a big person, but can be more comfortable to some (like me).
The time to solo a 300 will be less than that to be ready to solo an R-22, and if that time is less than 20 hrs, maybe doing it in the 300 or at least starting in one is better. If you're going to do your training where both are available, why not try each? You'll likely feel more in control with the 300 at first, but maybe later you'll have more fun with the Robbie.
As you can see from the posts above, everyone has their favorite. You'll have to try it out to know yours. Good luck!
