Truckracer hit on the reason for my 35cc preference. If you buy a 30 and at some point want to install it in something where you'd like a scale exhaust location, there will be no extra expense to buy the wrap around muffler for the side exhaust engine. One of those, that will last you a while, is going to cost you the 100 difference in the 2 engines.

The rear exhaust engine is also the more compact install, requiring less in the way of potential cowl hacking than it's side exhaust counterpart.

Of course if you never plan on using the engine for anything but a Stick, it's a moot point.