From my experience and reading around: (about the ICS, G&P and Guarder Kits)
G&P:
Holds Sub C Celled Batteries
Battery can't be taken out from the ANPEQ once assembled
is a bit big and unrealistic in dimensions
ICS: (have experience with)
Holds 9.6V 1700 MAH batteries
Battery can be taken out
Least realistic in appearance (seems like a flat box)
Mount is in the center, so not too prone to cracking (haven't seen one crack yet)
Guarder:
Holds 9.6V 1700 MAH batteries
battery can be taken out
realistic in appearance
Mount is on the front of the ANPEQ, which is more prone to cracking then others (I've seen a few crack during games)
Basically each has it's advantages and disadvantages from what I can tell. You in turn have to decide which pros and cons affect you the most and pick. Here's an article concerning these three Kits:
http://www.elementconcepts.com/articles_peqcomp.php Good luck!