I'd invest in glass first -- you'll keep that forever. There will be a new "must-have" body every year but the truth is, a skilled photographer with good glass can take great photos with any body.
It's cheaper to rent glass than it is to rent a camera body. So I would say go for the camera body. The fact of the matter is the camera, lens, and the photographer make a great photograph. I shoot with a 5D and a 50mm 1.4 and I love every shot that I take with it.
From the situation the original poster stated. It seems like he'll run into either one of these problems.
If he buys the 5D, he's stuck with the lenses that he already has.
- you can always rent lenses... I rent them for about $30 - $40
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If he buys the lenses, he's stuck with the same camera body.
- you can rent a camera.. the 5d in Kansas City rents for $250
My advice.. go with what you can afford and what will benefit you in the long run.
For LF, it's more about how big the image circle is, as that determines how far you can tilt shift, rise and fall the lens or film. Some of the "I shoot tech pan" 'blad crowd were obsessed with sharpness, but it centered more around film and developers as you were approaching the limits of what you could do (and for MF, of course the enlarger lens was as important as the camera lens for very high detail- with more wiggle room at the small LF sizes and the ability to contact print with the larger LF sizes.)