Well, I currently own a 1Ds and won't be switching to a 5D. For my type of shooting (travel, mostly handheld) the 5D is probably a better camera:
- smaller and lighter
- smaller batteries, meaning moore spares can be carried
- lower noise at high ISO and higher maximum ISO
- I don't really need the weather sealing and tank-like build quality of the 1Ds
However, these benefits are not enough to make me take the financial hit of selling my 1Ds and buying a new 5D. The 5D looks like it will go on sale here in the UK for approx £2500, and once it is on sale I'm fairly sure the price of used 1Ds will drop below £2000.
My 1Ds is still an excellent camera that produces exceptional images when properly exposed below ISO 400. I'll continue to use it for now and probably get a 5D Mark II in a couple of years time.....
If I was starting from scratch, I'd buy the 5D without thinking twice about it, and unless you REALLY NEED full environmental sealing I would suggest you do the same.