As you pointed out with your comment on paved runways, I think the choice of engine depends a lot on where you fly. I first flew my LT40 with 46LA. At our field, you have a fairly short runway and then have to make an immediate climbing turn to clear the trees at the far end of the field. You then have to continue climbing and turning to get above the trees on the downwind leg. Takeoffs with the 46LA were a little more "exciting" than was really comfortable given my experience (or lack thereof!). A missed approach was definitely pucker-time if you left it a little late. I changed to a 46FX and it made a world of difference. Takeofs were all of a sudden a lot more comfortable. Either will work. Go to field(s) you plan to fly at... there's almost sure to be an LT40 or two. See what motor they're using... ask the instructors which motor they would recomend with an LT40.