The W520 (W510 has similar or same board dimensions) and W700 have significantly different motherboards as illustrated by these (W700) photos (W510)
That said, though, there isn't a lot of difference between a W700 with a Q9000-series processor and a W520 with a 2720QM other than power consumption and heat, especially
if you add an SSD to the mix. So if you want a W700, get a W700; you can upgrade parts later as you see fit. And besides- graphics in the W700 series are faster than those in the W520 series; though the Quadro 2000M series approaches the FX2800M in speed, you're looking to spend another 250 bucks on that upgrade. The Quadro 1000M doesn't come close.