I don't know that LA and FX stand for anything in particular...probably just a way of differentiating the two lines of engines...

How the evolution and the OS46LA compare in terms of power, I'm not sure... but I have flown an LT40 with both an OS46LA and with a 46FX... the LT40 will fly on the LA, but I found that the FX was a better match for it... our field is surrounded by trees (actually make that TREEEEEEES!), so immediately after take-off you had to make an immediate climbing turn to get over the trees at the far end of the runway and continue climbing to get over the trees on the downwind side of the pattern... the LA, being weaker, made take-offs and go-arounds a little more "exciting" than was comfortable at the time. The FX made things much more comfortable.

Bear in mind, that this was relatively early in my flying... so things became "exciting" quite easily. If your field allows for more gentle climb outs from take-off, you can probably do OK with the LA...my particular combination of skill(or lack thereof) and terrain made the FX a better choice. What you might want to do is go to the field you'll be flying at and ask the instructors their opinion...

My opinion, if the evolution is roughly comparable to the OS 46FX, get the evolution.