As someone who went from an MX700 to an MX900, I can tell you that the MX900 is just as responsive as the mx700, except when coming out of sleep mode or idle, then it takes about 4 seconds to re-sync w/ the bluetooth hub. But as far as range, my MX700 worked about 6' away... the MX900 is guaranteed to 30', and mine works anywhere in the room, I love it. Same goes for the Bluetooth elite keyboard vs. the wireless elite keyboard.
If it's important to you to have that kinda range, go for the MX900. I'm using it w/ an HTPC, so the range is neccessary. Otherwise, don't waste your money.