For your main selection criteria (High ISOs), the 5D2 is the better choice. The 1Ds2 tops out at 3200. And at 1600 it isn't that great, whereas the 5D2 seems fine at 1600 and you can push it up to 25,600.
actually the 1ds2 stops at 1600
3200 is pushed (expanded ISO)...as of my trials with my friend's camera ISO 800 was the maximum ISO for "still decent result"...for me 1600 (limit) was way too "disturby". Mind you, I don't hate noise, I just don't like that digital look when you go on the extreme of the ISO range and you start to see that tipical digital type of noise...
So beware dear K-amps, 1Ds2 goes up to 1600, Whereas 5d2 goes waaaaay higher and (for me) 3200 is coompletely useful and utilizable!
Mind you, and remember, you can't truly and fully compare the two sensors because they are different...is like comparing the 5d2 with the 5d original...you can but there are 2 different results.
I'd consider the 1Ds2 if you are shooting a lot of activity or moving subjects (better and faster autofocus and 5 frames per second) or if you needed a rugged, go anywhere, reliable body. Plus it has a longer lasting battery and faster flash synch. Overall, the 1Ds2 is a better built camera than the 5D2, but then the 5D2 has more features (eg video) and is more versatile, better LCD, is llighter, and goes to higher ISOs. And if you buy a new one, it comes with a warranty. The 5D2 is the logical choice.
Here Hillsilly is about right...if you're lookin at the 1ds2 is mainly for its build quality and for AF...damn even a 5 year earlier camera than the 5d2 has 45 points...though it's not contrast detect but phase detect. Real world difference, I PERSONALLY DUNNO (meaning I hardly find the difference on the field)....technically speaking contrast detection should be faster, but here I venture in a world where there's plenty of people (also here on CR) which is way more prepared than me.
What I CAN say to you (and which I highly suggest you since the amount of money is the same) is to try for yourself...for me 1ds2 focus was unbeatable (vs the 5d2 off course) but then again, 5d2 for me is totally fine, since I have the 7D too and since, and BELIEVE ME, it's the user that can make the 5d2 AF work, not the camera itself...I've seen action shots made with 5d2 which where incredible, perfect. I too make panning shots with my 5d2 with no problem at all...it's just practice and knowing your tool
Last thing related to what Hillsilly said in the last part...I totally agrree and also consider this.
Will you find it easier to sell a second hand (almost new) 5D2 which has come through your hand as new? or is it easier to sell a third hand 1Ds2?!?
I personally would prefer to buy from you, in that point of time a 5D2 instead of a "beaten" third hand 1Ds2, isn't it?
Well, up to you then but if the 5D3 comes out suddenly in the middle of spring time and you abs want it, in my opinion you'll find a ton easier selling your 5D2 with a good retained value than a tripple used 1Ds2 to get faster money for the 5D3...Just my thoughts