I'd say get the Nexus S, its already getting Android4.0, and will get more updates as its updates come straight from Google, its GPU is also more powerful than the Incredible S's, has a larger battery, better screen and it has stock Android.
Want latest updates from google? Nexus S
Want to boot custom ROMs? Nexus S
What more vivid images? Nexus S
Want sharp text? Incredible S
Want solid Exchange support? Incredible S
Like arrow keys on your keyboard? Incredible S
Like long-press on every key for alternate characters? Incredible S
Like a coverflow style UI on your media player? Incredible S
The last four can be worked around by using third-party apps, but you'll need to experiment to find ones you like. If you're OK doing this, then the Nexus S is the sensible choice.