The OS .50 and Super Tigre .51 will both fit in .40 sized trainers with no problems. The extra power is nice for when you need to get out of trouble fast, like aborting a landing at the last second. I got an OS .50 SX ringed engine for my Avistar, when I first started flying planes. I figured I spend the extra $$$ up front to get a good quality engine, and just re-use it in my subsequent planes. So far it's made it through two crashes (dug it out of the dirt on that last one) and is now flying my SPAD very reliably, and with plenty of power.

I highly recommend the OS .50 SX for someone starting out, but if you can't swing the $$$ for it, the ST .51 ringed engine is about half the price!