Upfront, I make no claim to be an expert mountain biker or have expert knowledge on frames.
The only comparison I can make for the M390 (built up with complete SRAM XX) is with my previous 2010 GT Avalanche 1.0.
The GT was a 26er and the M390 is a 29er. The M390 frame is much lighter than the GT frame. The GT felt stiffer (probably because it was welded and built like a tank). The M390 does not feel flexy or weak. It feels it has just the right amount of compliance. Surprisingly the M390 feels easier to control twisting through trails despite the larger wheels and longer wheel base. The M390 has a large sized downtube and the option to go SS (which probably keeps it from being a weight weenie's people's choice at 4 lbs 6 oz for a medium frame).
In conclusion, yes the M390 is a great bike. IMHO if you researched your options with your budget well and still feel drawn to the M390 go for it. Build it up with deserving components and you will not regret.
The only complaint I have so far with the M390 is the cheap decals. I plan to remove the decals and keep a sexy bare Ti look. The Lynskey badge on the head tube and name on my seatpost is more than enough to make a statement to anyone who cares to look.
I paid $1800 for my frame. I thought it was a fair price for a Ti frame. I wanted the sexy helical tubes of the PRO 29 but also wanted the geometry for 120mm fork. The PRO 29 was really designed for 100mm XC racing, but some people put on 120mm forks and say it works even better. But I just wanted pure 120mm for my riding needs. The killer part of the PRO 29 was the price.. Almost $500 more than the M390.
At $1200 you are getting a 30% discount from my buying price. So I would say thats a great deal already.
I'm 5'10" with a 32" inseam. My M390 is a Medium. I am using a Lynskey Ti 27.2 x 400mm seatpost. The post is stretched way out. I consider myself an in-betweener in Medium and Large.
If you plan on getting using an adjustable (dropper) seatpost in the future definitely get the Large frame. If you want the lightest build you can do with the Lynskey M390 get a Medium.
My posture is slightly more upright because I use this bike as my all around do everything bike. I bike to work at least 3 times a week. If I wanted more race posture I could go with a flat bar, slam my stem to the head tube, and get a seatpost with a few mm of set back.
The best really is to demo both bike sizes first but I know that's not really an option for you. Good luck.
Here is my set up:
Lynskey M390 Medium
Rock Shox XX Revelation 120mm
Truvativ Stylo T40 Low Rise Bar & 75mm Stem
SRAM XX Drivetrain
Lynskey 400mm Seatpost
Shimano XT PD-M785 Pedals
Shimano XTR Rear Skewer
WTB Race Bronson 2.2 x29 Tires
Stan's Arch EX 29er Rims
DT Swiss 240 Hubs