I'm heavily biased, but would tell ya to get the M2 stumpy...
The M2 frames were an attempt to get a steel-like ride in an aluminum frame. They come fairly close to that. Compared to my father's Avalanche LE or my wife's Hardrock AX, my Stumpy rides very comfortably, with a good deal of shock absorption done by the frame. It is noticably more flexible than either the GT or AX if one attempts to flex the frame by standing on a pedal with the brakes locked, but it pedals with wonderful efficiency.
The frame was advertised as a 3.8 pound piece, with my 20.5" version right at about 4 pounds. Sensibly light, not stupidly so, IMHO.
The S-works versions were similar, but about a half pound lighter with butted tubes. The M2 Stumpjumpers used straight gauge tubing, except for the externally butted seat tube.
18 years down the road, my '95 is still going strong, rather stronger than I am. It's been ridden, XC raced, recreationally downhilled and generally been a faithful companion lo these many years.
I've ridden, briefly, a few Cannondale hardtails (all BITD). I've found the ones I tried to be excessively harsh in the rear, compared to my stumpy, but not so harsh as Dad's GT. Never really went for C-dale's proprietary front suspensions, they make upgrading a bit less straight-forward. Worst, to me, is the looks. Trying so hard to be different.... Not that there's anything wrong with that, if it turns your crank.
Link to my bike in my siggy. Either choice you make will probably do you proud. A taste thing.
J
