Thor,
If the budget would stretch to the 1DS2 I would go that way. But when it came down to either a 1DS or a 5D, I chose the 1DS. To my eye, the 1DS files have something of a unique look. Some call it a "film-grain" look, and I think that is a fair description. At low ISO ( 50-200) it's just about total perfection, lovely detail and stunning color. At ISO400 it starts taking on a bit of grain look in saturated colors, especially blue skies. I love using it for landscape work and for portraits in the studio.
Some folks would argue that high ISO is not really very good though, so the Mk2 version would probably be a better choice if that's important. But you mentioned using it at lower ISO, so I think you would like the older version fine. And at half the cost ( 1DS vs 1DS2 ) in today's market, that leaves money for another lens, studio lights or whatever. They seem to have stalled out at around $800-$900 for the last year or so. You could probably pick one up and use it for 6 months and then sell it on for little loss if you don't like it. The same could probably be said of the 5D as well, as far as dollar value....they seem to be stuck right around a the $1K mark.
I just like the feel ( among other things, like AF and AE accuracy ) of the one series, and since I use a pair of 1DMk2 bodies as my primaries, it's nice having the same control layout on the third body too.
