I was sort of unsure to post this becuase its still pretty early. IE I just figured out how env_sprites work in hl2 so leave me alone about the lack of sprites
Ive used Hammer and Radiant before. I'll make a quick comparison.
Quark has tons and tons of features and to me I feel like I can get things done a bit quicker but there are a few things that cripple it.
Bugs too many to list it is much better than it was. the last time i used it I couldn't do anything without it crashing and reverted to an earlier release they've fixed a lot in this one.
Shape's almost are never correctly aligned to the grid if you move them around or spin them this gets annoying quick and really screws up the maps. texture window crashes a lot on me. when cutting brushes it messes up regularly. You can search for invalid brushes but its not automatic.
Hammer and Radiant would probably be better for the hardcore level designer