STI's, EVO's and 350Zs??? C&D must be smokin crack! I can ALMOST see the first two... only because all 3 cars are 4 door sedans with awd and a lot of power. But that's basically where the similarities end. As far as the 350Z goes... hmm.. let's see... ok, they're both fast. umm... they're both cars. ok, that might be it.
Seriously though, like Newf said, it mainly competes with the LGT. I'm not sure I'd lump it into the same category as the Spec-B though... but it's, at the very least, debateable.
Bottom line. STI's & EVO's are for "enthusiasts" that value speed, handling and "look at me" styling first... and basically everything else second. I think the interiors of both cars are better than a lot of people give them credit for, however. Granted, there's not much to them... no power seats, no fancy keyless entry/start, illuminated dials, navigation, leather seats, etc. Those are the types of pseudo luxury things yuo can expect on the MS6 (GT anyway) Without question, the STI has a far better interior (thanks to Saab's help in redesigning it for the '05 model year) But neither are as low-rent as some might suggest. In comparrison, the MS6 (while having a ton of nice features like the ones I just mentioned) is in no way as high quality as many of its competitors. After recently test driving one, one of the things I couldn't get over was the chintzy plastic interior (namely the air vents, dash storage bin and center portion of the steering wheel.) the LGT, while failing to capture the same aggressive stance of the MS6, has a very well thought out and put together interior. It, unlike the MS6, can at least hold its own with the higher-priced competitors (think Audi, Lexus, VW, etc.)
What it comes down to is the test drive... I'm assuming, because it's so different, the 350Z is NOT on your short list. Do what I did and test drive the LGT, STI, EVO and MS6 back to back (if at all possible). Nothing I or anyone on this forum... or any magazine will be able to provide you with the level of insight you'll gain from that couple of hours. There are some considerable differences between the aforementioned cars so only you can decide which one fits your needs.
If you're leaning more towards having a "gentleman's EVO", unless your plan is to class the STI or EVO up a bit, skip those completely. In this price range, unless you want to sacrfice performance, the LGT and MS6 are really your only options.
Here's a couple articles you may find helpful.
http://www.roadandtrack.com/article.asp?section_id=31&article_id=3280
http://www.motortrend.com/roadtests/sedan/112_0602_midsize_awd_sedan_comparison/index.html