You should try both and see which one works best for you. rom manager allows you to flash clockwork or alternate(sprecovery). personally I had problems with sprecovery not flashing roms properly or at all in some cases. after switching to clockwork, all was well. obviously your experience may be different. give each one a fair try, and then make a decision.
Never had a problem with either, both can do USB mount (CR buries it in the partitions sub-menu), CR can do a zip update without the file being named update.zip.
CR has worked very well for me, for both standard .zip ROMs and Nandroid restore ROMs.