I own both the Walther , the Steyr M and the MA1.
The MA1 has a lower bore axis, the felt recoil is less than the P99. I can make follow up shots a lot faster with the Steyr than I can with my P99
Grip, slight edge to the P99, not by much
accuracy, It's a toss up
Ease of take down, Steyr
Trigger, AS trigger is better, Steyr is a great trigger for a "Glock" style trigger. Six of one half dozen of the other. Depends on the style of trigger you want. If you want a non Da/Sa trigger Steyr is the one to get. very short re set, almost no take up and crisp like a 1911 (just heavier)
Construction, I would say the Steyr is a bit more robust, everything is mounted to a steel sub frame. No pins going through polymer . It is also way more modular so service is going to be easier. Steyr has a built in lock I don't use mine at all.
Steyr in 40 has a fully supported chamber. I think Walther does also. Barrel is conventinal rifling so lead is OK, don't know about the walther on this.
Steyr has a standard magazine release, this may not mean anything to some. For me as a Police Officer it has a "standard" manual of arms in the event I need to switch duty weapons.
Both are tennifer, the Walther blued slide is nicer than the Steyr phosphate.
Current MA1 has standard tac light mount as does the newest Walther.
Walther, more choices and easier to find holsters for. The MA1 only has 3 current makers and none of them are "gun show" types. This is changing. night sites are going to rare for the Steyr. The trapazoid sights are really great and are pretty accurate also once you get used to them.
Steyr is lower priced than a Walther by around $100.00 or more.
Take your pick. Both are excellent weapons.