There's strong arguments for better lenses, especially as the theme of the day on this forum is the limitations that lenses impose on quality. Furthermore, there's a lot to be said for the general principle of investing in value holding glass rather than camera bodies with an upgrade cycle of 18-24 months.
However, having said this, in your case I'd recommend the 5D.
IMO a D Rebel is starting to look strained at A3, I appreciate that many satisfied D Rebel owners will vigorously dispute this, and with the right subject and right technique I freely concede that it's capable of producing a satisfying A3 print. But absolutely everything has to be spot on, which in real life is rarely the case.
I use a 5D along with a 1Ds Mk II, and for most shots printed to A3 or smaller, I can't really tell the difference. For hand-held photography I don't believe there's any difference to tell! The results from the 5D also stand favourable comparison with the Hasselblad medium format film shots I've taken.
In other words I believe that with the 5D the digital market has arrived at an important milestone, a (relatively) affordable full frame camera that's capable of fully exploiting available printers and lenses. It's a camera that will give many years of service in the upper reaches of the quality spectrum, and allow you to pick up used or discounted prime lenses from the huge reservoir of EOS optics.
Just my opinion.