There was already a detailed discussion of the flavors of Pentium IIIs starting with the 500 mhz. I'm not sure what the best keyword would be to search under.
Anyway, IIRC, they are all flip chips now so it doesn't matter if you use slot one or socket 370. Early on, the slot one were all Katmai and didn't cool as well. They were 512k L2 cache too. I think most all are now L1 256k cache (on die) and are "core up" for best cooling. At the start of the FC-PGA, only the socket 370 (FC-PGA) were flip chips, so you had to buy FC-PGA and avoid SECC slot one - that isn't the case anymore.
The FCPGA (socket 370) format was started because they were less expensive to manufacture, but since slot one motherboards were so prevelant, Intel decided to keep the slot one version alive improve them with SECC2 (flip chip) with the newer faster P3s. There is no particular advantage nowadays to either slot one or socket 370. IIRC, they are all flip chip coppermines and even the cache may be standardized to 256k L1 on all of them.
"your next processor probably wont be either slot1 or socket 370."
I doubt that... From the sounds of things, the P4 isn't going to sell like hot cakes for a while. I think slot 1 and socket 370 will be with us for another year, maybe longer, that is unless you want to go out and lay down big bucks for some kind of hollow bragging rights. The Athlon and P3 based computers will be the best value/performers for a while longer over P4 or something else.
Get whatever CPU fits your needs. If you have a slot 1, go with that or visa versa. BTW, if you want to overclock, stay away from the B variety of Pentiums made for the 133 FSB. There isn't much head room for OCing. Get the 100 mhz FSB versions for sure.