rescomp is the tool included in SGDK so if you want to follow pure SGDK code, use it
and from what I know, it uses res file (to not mismatch old genres' rc file, which was previously part of SGDK)
GenRes generates resources as defined in the
genres.h
it's up to you to make your own resource player but a lot if already written on the sample, compiled using SGDK so....
GenRes and rescomp have some similar syntax
GenRes2 uses another one, to make it more configurable and suits more case
Stef will probably give you more support than I'll be able to give you (I think help and sample could help a lot but....not every situation and possible bugs)
GenRes will ask you more work than rescomp
so see with Stef if a Linux version exists (or source file) and compare the available tools with your needs then go !
I think moving from one to another isn't something easy to do so take the good one now !