Having owned an 08 Evo X and having driven the 6mt 370z and a 6mt (with the 3.42 rear diff) 5.0 mustang, here are some thoughts:
First if you are in socal and are interested in driving/purchasing an Evo, look up Sam at Huntington Beach Mitsubishi. The guy is plain crazy (in a good way) and will beat other dealers' price by a ton. I got my IX and X brand new from him - the X was an 08 with nav and was IIRC 31,000 out the door with LA county tax rate. Look up socalevo.net and you will find him relatively easily.
The X is very fun, though much less raw and more civilized than the IX. One major downfall in the X is the trunk size, it's ridiculously tiny for a 4 door sedan. Turbo lag is improved in the X but it's still a huge pain especially in LA traffic, and ESPECIALLY in summer heat. Basically everything will rip you a new one off the line unless you are doing a full launch which is just ridiculous. The gearing is very short so expect to shift a lot though that was a plus for me
Interior is definitely quite cheap and WILL rattle - it was much better than my IX but it's still pretty bad. I owned both my Evos from 4 and 7 miles, respectively, to about 30,000 miles. Both my Evos had basic bolt-ons and custom tune making around 320whp and was very fun up to about 110mph (top of 4th gear), after which most performance cars like the 370z and especially the 5.0 will walk away from the Evo relatively easily. If freeway runs are your thing the Evo is NOT a good car IMHO because it was never designed for it. Combining its brick-like aerodynamics and its super long 5th gear there are many other better freeway bombers to choose from. Sure you can slap on a huge turbo and deal with massive turbo lag and try to find better fuel than the 91 piss we get here in cali but that's just not my cup of tea, even after having owned 2 Evos in 4 years and having had different cars in between.
The Evo is based off of a rally car and is geared towards tight road courses with very short gearing with a peaky power output. It's mad fun carving canyon roads and that's what I loved about the car. The steering feel is excellent but the car understeers quite a bit esp. compared to a wider and shorter F/R (front engine RWD) car like the 370. The grip is insane but it does not feel as "natural" as a FR car would going around bends. At some point it becomes relying on the car's silly grip (until the sticky factory tires run out, which mine did after 8,000 miles) rather than the car's balance/center of gravity/etc. to achieve crazy performance figures. YYMV but after owning 3 turbo AWD cars I am ready to move on to an n/a RWD car - no turbo lag, much more natural handling, and going sideways at normal speeds are all things I am looking forward to in the 370z.
The mustang surprisingly did not have much low end torque IMHO, though i'm sure the 3.73 (iirc) rear diff. probably would have transformed the car. The car sounded good, but the vague steering and boat-like handling feel (i'm aware it's very competent around bends as it posted similar lap times as the e9x m3) and the interior - the interior was the deal-breaker as I just couldn't get over its design and layout. The coyote motor responds very well to mods and basically the Mustang's appeal to me was that it's a 30k M3 in terms of performance. However, the driving FEEL is a much different story as the few e92 M3's I drove (DCT and 6mt) just offer an other-worldly driving experience that a car like the Mustang, even with matching performance figures, cannot begin to compete with IMHO.
To me the 370z offered the best driver involvement and driving experience among these 3 cars. I would have opted for a 370 back in 08 if I did not need the Evo's 4 doors and if Sam did not give me a great deal with 0% apr.
If you are serious about the Evos look up Sam, you can also just call or text him 714 493 4163 - I don't get anything out of this, he's just one of the rare dealership guys that actually don't pull any tricks on you. I was out with my IX and X in less than 30 minutes after talking to him on the phone, and have purchased 2 used cars from him as well.
Good luck
Edit: sorry for the horrific grammar as it's quite late here and I'm supposed to be working