Weight wise, they are similar, or at least very close (The G700 could be slightly heavier due to the battery). They are shaped similar and both are good sensors. I have tested what looks to be the performance MX at Fry's, and it is a little different than the G400, but not a lot. It is much closer to the older MX 518 in shape, if not the same. (the lowest point of the mouse doesn't flare out nearly as wide like the Performance MX). The G700 looks more like the Performance MX, though in the hand, they don't feel much different (the wide base may be the only real difference).
The G400 is much cheaper, the G700 has more buttons, which I personally prefer. It allows for more hotkeys on the mouse, freeing up some of the duties of my left hand.
If you aren't one to create profiles for hotkeys layouts in games, you may prefer the G400, as you'll have to program two of the 4 buttons on the thumb side of the mouse from within Logitech's software for them to be useful (I make my default profile shift and ctrl, so I don't have to do it for every game and it can be mapped in game to shift or ctrl).
If the extra money is tight, you'd be happy with the G400, but I personally buy mice to last a few years, and I think spending a bit extra for what I really want to be worth it.
I might also mention that for the most picky, highest performance gamers, they often pick the G400 due to the Optical sensor which is supposed to be the most accurate. Yet a lot of professional gamers still use mice with the same sensor as the G700, so it obviously isn't a huge difference (I can't tell a difference).