My entire pistol collection consists of a Steyr M9 I bought from CDNN last year, and a CZ-75B (of which the Witness is a copy, as Only1Asterisk says). They're both great range guns for me.
By far the best way to make this decision is to go to a gun show and compare the feel of the guns in your hands, the different trigger pulls, and see how you like the triangle/trapezoid sights and the trigger safety on the Steyr. I think the steel frame of the CZ gives it a more balanced feel than the Steyr. I have long fingers, so my CZ with Hogue rubber grips fills my hand a little better than the Steyr. The Steyr is slightly smaller and lighter, but not by that much. I definitely like the round grip of the Steyr better than the square grip of comparable Glocks.
Go to
www.thefiringline.com, do a search on the Steyr M9 in the auto pistol forum, and read about the controversy over whether the Steyr has a firing pin block or not. Make up your own mind on this. Also search here in the High Road for the lastest information.
For maintenance, the CZ and Witness have a definite advantage. The repair and parts situation has always been iffy for the Steyr. Diamond Trail Shooter's Supply is supposed to carry spare parts and do repairs now, but you should call and verify this yourself. You will be able to get a much wider variety of holsters, grips, sights, spare mags, new springs, etc. for the CZ/Witness.
My Steyr has been 100% rock-solid reliable, even though the magazine is slightly loose when it's inside the gun. This reliability may be due to the fact that that you can only buy factory mags for it. My CZ has been mostly reliable with factory mags, but has had major problems with aftermarket ones.
Both guns are more than accurate enough for practical shooting purposes.
If you're willing to pay a little more for a gun that might save your life one day, take a look at the CZ P01, a light, compact CZ with an alloy frame. It's a new model that has gotten great reviews so far. It sells in the $400+ range. I'll post more on this if I can think of anything else hat would be helpful.
With a proper stance, there is virtually no muzzle flip for me. My M40 loaded with warmish "carry" ammo still flips less than my 9mm. 
