Both are great guns, although they fill different slots in the armory. The 92FS is a full-sized duty holster gun, the P99 is more of a compact CCW piece. Both are very accurate, the P99 is much lighter and easier to conceal, the Beretta has less felt recoil and a longer barrel. Trigger on the Beretta tends to be a bit better, but the Walther trigger is as good as they come on plastic guns.
I have a Beretta 96 Inox and a Walther P99 (non-QA), and I wouldn't want to be forced to pick one over the other. For carry, the Walther is probably better for many people...it weighs much less and is more CCW-friendly in its dimensions. OTOH, I also carry my 96, and I found that its length actually makes it better for IWB carry.
I'd keep the Beretta and put the Walther on layaway. The deal sounds pretty good, and I know how strong the "new gun jones" can be, but you'd trade a fine pistol away. The Walther is an equally great choice, and it really boils down to whether you'd regret trading your 92FS too much.