RE: What size StiK?
It kind of boils down to a couple of thoughts. If you want to use the engine from your trainer or buy another. Also consider the fuel bill with the larger engine. You have already established a good track record when you started with the LT-40, and picking a Stik for a second airplane nearly puts you into the genius class.
If you get the 4C engine, I would advise you to also buy a tach to set the needle with. While I personally prefer 2C engines, setting the engine back 300-500 rpm from peak seems to make a world of difference when operating 4C engines (your mileage may vary).