Personally, the only time I'd put batteries on the stock is if I had a balance issue. Magpul is developing an attachment that holds batteries to the side rails.
I've never had the oportunity to train with a weapon with bateries mounted in the rear. Is it faster than say in a LBV?
The Magpul will give you many more preset locations to adjust the stock to the 'perfect' length for you. I have two, and there is absolutely no play whatsoever in it when locked. Unlocked, I don't want to say it wiggles, but you can tell a difference in how firm it is.
Still stronger than any collapsible stock I have ever used. I haven't gotten to use the Vltor as of late though.
Magpul is developing strike plates for the buttstock. I've heard of a steel and rubber one in development. I'd feel comfortable beating someone down with the stock as is.
If looks play any roll (which they do) the Magpul is better looking IMO. I don't know what type of work you're in, but a bad guy sizes up the opposititions tools. This thing looks like you just robbed Robocop and took his shit, expecially if you have a SIR rail up front.
I'd piss my pants if I was on the down wind side of it.
-Steve