Actually it's not mine, and it's a small. I have ridden both a medium and a large and think I could deal with either. I'm 5'11" and have fairly long arms. The medium is a bit short at 23.4" tt and the large pretty long at 24.4". I guess I'd go for the medium and run a 110 or 120 stem on it. I did not ride the sizes back to back so it is hard to compare. You can draw your own conclusions for the result of the longer wheelbase and stuff I guess. I just went to a shorter stem on the Spot (100 down from 120) and got used to the fit pretty fast, and like it better for descents. The climbing position suffers a bit though.
I was just emailing with Squeaky over this: the 575 and Spot offer similar handling overall, but the suspension feel does differ. The 575 is a bit stiff around sag, and then gets nice and soft as the suspension compresses. This is a function of the linkage rate changing. It is a falling rate bike overall, and so the shock's damping and spring feel like they have more effect higher in the stroke. It makes for a very good pedaling bike that uses it's travel effectively on bigger hits. It is a little platform-y feeling at sag but really sinks into the travel when a hit demands. That seems to be how it pedals well and yet feels plush. The change in linkage behavior through travel can be a bit of an odd sensation at first. A little like hopping on a vpp for the first time.
The Spot seems to have a more consistant and rising rate linkage. It rides more supple at sag (bob-y, some would call it), and then resists deeper compressions in a very predictable and evenly increasing way- progressive. You don't feel any transitions in suspension action as it compresses.
I guess the 575 feels a bit snappier and racy, the Spot a little more trailbike-ish and stable. The frames are actually about the same weight (within about 0.25# or something). The 575 is noticeably louder bike from the big, thin tubes acting as a sounding board for any chain slap or knocks. The tubes seem absurdly thin when you flick them with a finger. The cable routing on the 575 is sort of weird, but it works well. The 575's standover is excellent. The RP3 compliments each bike a lot and at this point is the shock to beat. The 575 uses a special 2.25" stroke RP3 so it has roughly the same leverage as the Spot.
You're a big boy, I can't picture you on a medium, but that tt is closer to your large XCE's.
Shopping?