ORIGINAL: forestroke
i'd say that since it is not question of which you buy, but which of those that you have should you use, i'd suggest the 2 stroke for all the reasons above. it makes no sense for you to use the 4 stroke when you have a more resilient 2 stroke lying around.
although i've seen a lot of four stroke trainers recently, i still think that a two stroke is better for the following additional reasons:
1. they are lighter - for the amount of power they produce, two strokes are lighter than their four stroke counterparts so less wing loading = better trainer or much more power. the 46 will be significantly more powerful than the 52 for about the same weight. the irvine is really about the power of a Saito 82.
2. they don't have as much torque - undoubtedly you will be aborting a lot of landings. the 4 strokes tend to torque the plane when you put the pedal to the metal. but then with the 52, it may not be that different. with a 82, you'll definitely see the difference.
3. slightly shorter props - well this might not be true with the 52. but with a power comparable 82, the props will be longer and more expensive
4. everyone and their mothers have run 2-strokes so you'll get a lot of support (wanted or not!) at the field. 4-strokes tend to be less understood.
but as you can tell from my handle, i'm a four stroke addict and really am running only one two stroke engine. it is also the sound and the mechanical complexity of the four stroke that won me over.
I'm going to agree with forestroke and the other pro-2 stroke posts. Yes the 4 strokes sound great so instead of risking it, save it for your second airplane. As forestroke stated 4 strokes do tend to torque the plane and when learning how to land you definitely do not want that happening. I learned to land first by doing practice approaches, then learning how to abort a bad landing before I ever made a true landing attempt. The airplane rolling left at 4 ft off of the ground when going to full throttle is not a tendency you want in a trainer.
Like forestroke, I don't know how much of a difference the 52 will be torque wise but he does make a lot of other valid comments. Power will be greatwe with a .46 two stroke, you won't be worrying about prop clearance and you'll get tons of help.