Why do you want to upgrade? Is there something the 5D isn't doing for you or do you need larger prints that may benefit a little from the extra megapixels? I doubt you'd see any difference at 8x10 at a typical viewing distance, even if you could see it with a loupe using a higher end printer. lol
For all practical purposes, it's a 5D with 10MP sensor. There are some other changes (for example, different metering, faster write speeds to memory cards, new LCD, DRO). They changed some of the mode dial layout (they replaced the 5D's dedicated White Balance dial with one designed to select from different functions, versus being dedicated to White Balance, and the ISO speed button got removed from the A100). But, most of it is very similar.
But, from an image quality perspective, unless you need very large prints, I doubt you'd see any difference (and some people like the 5D colors better).
If you need to shoot at higher ISO speeds often, you'd probably see a degradation in image quality for those. Now, the A100 is more sensitive than rated. It's ISO 800 is really ISO 1000, and it's ISO 1600 is really ISO 2000, according to Phil Askey's review. But, it's a noisy camera if you tried to use it's highest setting.
You could always keep both (your existing 5D and a new A100), using the 5D if you needed ISO 1600 or 3200 for anything. But, I really don't see a big benefit with going that route unless you need the extra pixels for something.
I'd probably wait until Sony has an upgrade for the A100 available if you don't want to go with the A700.