to summarise what has been already said, you should go for an SD build if you want to dual boot between WM/WP7. Its also a good choice if you like safe and easy installation. However, the SD builds are slightly slower compared to NAND builds and booting time is significantly slower as well. That said, if you want to trade-off the benefits of the SD build with the performance boost of the NAND build, you should flash the NAND build. It all depends on what you want on your phone. My advice however,is that if you want CM builds, the performance difference is little between SD and NAND builds. However, if you want a Sense build, NAND will be noticeably faster and better.