Both bikes are good, the 300 is a bit better all around in my opinion. I'm not sure if you can legally upgrade to a slip on or a full exhaust on a 13 bike though. I never actually tried riding a 300, but I can imaging how fun the extra power can be. Plus it it's fuel injected bike with a wider rear rim. You can't go wrong with either bike, both are fun and lightweight.If I were you, I would just buy a used 250 and maybe get a 300 later down the road.
1. SLK 230K AMG SPORT
2. VW RABBIT 2.5
3. 08 Ninja 250R Track Prepared