I have a 600D right now, so we are in a similar situation in a way. Now it of course comes down to economy, but instead of trying any halfway solutions - a 50D in crop mode would be quite fine actually - I will just save up and buy a 5d3 when the time comes.
Of course a 5d2 is a far more affordable solution (half the price used here in Denmark) I want to be able to record good quality stuff outside of crop mode as well (there are a few ways to piss me off, and moire and aliasing in my footage is one of them...) so I rather save longer, but buy the 5d3 then. Also, a 5d4 is rumoured, GH4's and A4s' flooding the market, so a drop in 5d3 value in the next half year or so is to be expected as well.