I did the same thing, though I didn't mean to. I won a Glock in a shooting contest and decided that I could get more cash selling the P99. So I kept the Glock as the bedside gun.
Anyway, here's my advice. Get a Glock 22 (full size) or G23 (mid or compact size) in .40S&W. Then get a Wolf 40/9 barrel, around $104, for it. Now you the best of both worlds and can shoot either 9mm OR .40S&W.
Mine came as a package. Probably put together by the distributor. It included the G23, the Wolf 40/9 barrel, 2 each 9mm and .40 magazines and Triijicon night sights for $489! Even if you have to buy the parts separately it's a good deal.