You'll probably get some technical answers based on whatever the respondents chose at the time. You want to look at whether somebody has had a bad experience with either of these.
Unless you're going to be using huge assemblies with surface rendering, teamcenter and machining probably all at once, then you're talking about two of the latest and greatest processors both of which meet or exceed the requirements for NX-5.
I bought a Conroe 6700 system about a year back. Still going quite strongly enough for my moderate needs. Elsewhere I have experience of some sites where their hardware was up-spec but about a year older. When I've had occasion to push it to the limit performance naturally suffers on any system under load, but generally the systems cope without incident. So if you can draw a comparison based on benchmarks then my system would be about twice as fast as those.
The two processors you have talked about are both quite close to one another near the top performed in benchmarks for the latest crop of processors. The Q6600 is rated to be better than twice as fast as my processor.
Since there is not that much to pick between them the main difference may be price. According to what I'm seeing there are a couple of versions of the E6850 processor, a base version and an "extreme" one. The "extreme" is a bit faster than the Q6600, and it is based on the same Kentsfield architecture. The basic Core 2 Duo E6850 is only half as fast. On price the basic E6850 is a bit cheaper than the Q6600, but the E6850 on quotes I have seen is way overpriced.
If you can afford it probably the Q6600. I've had a good run with Intel processors over the years. I generally try and pick the next to latest model that I can get at a decent price point it is always a battle to weigh price against obsolescence.
Happy hunting
Regards
Hudson