Lots of options here, Sodapop. Here's a few ideas for you...
One, depending on what regs your local ranges promulgate, a 18-20" bbl may not be allowed there.
If it is,use that for clays. A short bbled 870 with an open choke does a very good impersonation of a quail gun, BTW. And if you learn to acquire a little clay Frisbee moving at 35-50 MPH and blow it to smithereens, using that same shotgun in a crisis means you've got a nice advantage on much larger, slower targets.
Two, most folks prefer a longer bbl for longer sight radius(Silly, we POINT shotguns, not aim them),more inertia to keep the swing swinging, and cut the blast. Scores at trap with Frankenstein, my 21" bbled parts gun,run a bird or two behind those with my 870 TB, with its 30 bbl. The swing's the things.Best of both worlds is easy, get two bbls.
Three, a good cost effective way to do this is to keep one's eyes peeled for used 870s, including the nigh mythical police turnins. If the 870 you buy has a short bbl, buy a long second bbl, like a 28" job with Remchokes. If you find a nice long bbled 870, most police style gunshops have 18 and 20" plain bbls excellent for crisis management at low prices.DML had some for sale recently at great prices, maybe he has some left.
As for buck, like Justin says, it takes time and testing to find the best for YOUR shotgun. Same wiht chokes, they're interelated as West VA hillbillies.
HTH....