The Nova will be a lot cheaper than the OS, and better made. Most OS is made in China now, the Novas are still made in Italy. Only real downside to a Nova is most are Turbo plug engines. Not all bad, turbo plugs last longer usually.

2.5cc Novarossi (Rex) engine for Revo $124. http://novarossidirect.com/N25SCRT-R...VO_p_3336.html
Add the pipe, manifold, and coupler for $38 and you have $162 for a complete engine and exhaust.

OS .21TM with Revo manifold is 270!! http://www3.towerhobbies.com/cgi-bin...&I=LXRTM2&P=ML
Add on a better pipe for $50 and you have $320 invested. Seems to be a no brainer to me. Legendary Italian quality for half the price.

Before anyone nitpicks, I know this is a apples to oranges deal comparing a .15 to a .21... But really it's a small cased .21, so I really doubt it will pull that much harder. Either engine is likely to out power a 3.3 pretty easily, but my money would be on the little Nova. Imagine the look on your buddy's face when you tell him it's only a .15.... LoL.