As far as which groove you seat to, it depends on what handgun you are using. For instance, Ruger's have long cylinders and you can seat the 300 XTP out farther than some other handguns. The COL for my 240 and 300 grain XTP's is 1.600". I have not worked up anything for the lower groove yet.
As for taper vs roll crimp, well......I guess you can get by with a taper crimp, but 99.9% of the rest of the reloading world uses a roll crimp.