I suggest you to try by yourself the different options for key caps, because as many have told you is a matter of personal preferences.
To add some information over what have been said, check the wiki's on materials and printing for caps:
http://deskthority.net/wiki/Keycap_constructionhttp://deskthority.net/wiki/Keycap_printingIn summary the main variations for caps are:
Materials: PBT, ABS, POM, a few are made of non ferrous die casted metals, but they are expensive and do not have legends, they cannot be considered main stream caps.
Printing: Pad printed, lasered, dye sublimation and double shots, which is not a printing process, but a molding technique that allows to have plastics of two or even three different colors in the cap surface, most of the DS are made of ABS, but there are a few made of PBT, I just found mods made of PBT double shots, I have not seen a full set made of PBT with double shot legends yet, maybe the gurus at GH may enlighted us.
Profiles: sculpted, spherical, cylindrical and flat.
Wall thickness: Thin - like WASD caps, medium thickness and thick.
Availabilty:
For double shots the sources are donor boards by cherry and Signature Plastics, recently GMK began doing businesses with GH with the mods group buy, not for full sets yet, sherriton and his multiple sites may be an option, however I have no experience with him, there are rummors of outstanding orders, payment problems and deficient communication.
In my short personal experience the actual sources for double shots have been the marketplace at GH and eBay with donor boards, I went into the group bay with GMK as well. I prefer the profile, material, thickness and texture of original cherries, so if I buy another set it would be either from a donor board or from GMK through group buys.
PBT with dye sub are available as well in GH market place in different colors and fonts. I have one PBT dye sub set and while I liked it, I prefer the cherry double shots by all means.
My grain of salt.