let's look at it from this perspective.
with the DX, when it came out, it was $170-180. it came with the back pack, sticky bomb gun, a PERS head, a light up sonar head, batarangs, 3 changeable mouths, two belts-both magnetic, a bunch of hands and other accs (sorry, my mind is drawing a blank)....
UNFORTUNATELY, his necks, both the reg and light up, are a tad long and a bit on the slim side. (this is where modding comes in, but more on that later)
for the medicom ver. when it came out, it was $150-170.
it came with a set of hands, i forget how many, two batarangs....i think that's it right?
ok, sculpt wise, both are great. HOWEVER, medicom you need to either repaint it yourself or send it to someone to paint for you (if you don't know how to paint....like myself).
THIS is where modifications come in. there's nothing wrong with modding a figure that YOU own. some felt the medi has a better sculpt, others feels the DX has a better sculpt. it's all opinion and it's all subjective. the only mod i did was using the rubber neck the medicom ver used. making the neck on the DX ver look fuller, shorter and more accurate to what we see in the movie.
for the DX ver, i GUESS it's not really that necessary to do any modding. all you have to is tilt his head at a really low angle, and the long neck won't stick out as much. but...if you like to pose your figure in tons of poses, always having his head tilted down isn't all that great. i guess if modding it will make you happy, then go for it...it sure made MY DX bats that much better.
so...both are going for crazy prices, i already listed the things that came with both figs.
all in all, its ultimately up to you.