That's like comparing apples and oranges. SRAM is where the game save goes, and NOR is where the game itself goes. It might be PSRAM you're thinking about, in which case it's sort of like NOR except that it disappears when the DS is turned off. It's much faster than NOR when you write the game into it, though, which is an advantage. However, games flashed into NOR can be booted again right from the DS's firmware, but they take ages to write games to. Slot-2 flashcarts tend to have 16MB of PSRAM and 32MB of NOR, so the bigger games have to be flashed to NOR, but the smaller games are usually just flashed to the PSRAM.