Commuting - Rack Questions

I've just recently started commuting to work on my old Specialized Hard Rock. I typically have my laptop backpack with the computer, change of clothes, and food stuffed in it. I would really like to take everything but the computer out of the backpack and put it on a rack somehow.

Eventually I think I want to get myself a Kona Dew or something similar. Could I mount a rack on my Hard Rock and easily move it to the Dew when I do get one, or do I need to get a specific rack for specific bikes?

I also don't know if I should get a trunk bag or panniers. I assume trunk bags are just easier to take the whole thing off when you lock up the bike, but panniers give you much more storage room. is that the biggest difference? I thought a trunk bag would be fine for clothes, food, phone, wallet, keys, etc...

I've looked a little bit at nashbar.com and prices seem to be quite lower than the LBSs so I wonder how difficult they are to install if I were to do it myself.

Thanks for any help!

Easy to install, yes, and pretty much fit any bikes, though there's always an exception for everything. Name brand racks seem to have more adjustments.

Rack bags are nice for convenience and aero. You can also get a laptop backpack pannier or laptop briefcase pannier and put everything in it, very slick and easy. Google briefcase pannier for lots of options. Or if you like your backpack it may be easy to mod it into a pannier with a few good quality caribeaners at attachment points that keep it out of the spokes and out of heel strike range.

I have a rack made by Topeak. I like it because the rack and bag use a track system, so the bag just slides on and locks into place. They are black nylon and not the sturdiest material, but not bad.