I would buy any instrument that's reasonably priced asap... Ps3 strats are going as high as $300 online right now. The Mustangs are cheap because apparently nobody's bought them. Trust me when I say stay away from the IONs - save the extra $$$ & buy a cheap e-kit and a MIDI Pro adapter. Unless you're not planning on playing pro drums.
As for dlc being compatible on the next consoles... If you're playing on Ps3 I'd say the chance of that is almost exactly zero. Well, there's a chance that dlc (or any digital content) will work but the disc would have to run in order for the dlc to be worth anything. Due to the Ps3's C/GPU & OS architecture I don't see it happening. XBox probably has a 50/50 shot. We'll find out in March or April.