i personally think the m16 grips are actually more comfortable, that being said you can't put a new grips like dye stickies on it and in the long run its worth getting the 45s, as there more upgradable and easier to take of imo
If you can live with the M16 grips then use them. Personally I like the M16 grips...on an M16. Not on a phantom. The ergonomics are not right for me, the angle is too steep.
If you are used to the 45 frame then you might find them a bit weird. You are probably better off getting the 45 frame as you will already be used to it and it will put your hand at a better angle for aiming the gun.