Dome-membrane. I find the MS 4000 rather mushy.
Dome-membranes vary a lot. You've got junk $3 featherweight boards with terribly mushy keys (my library has them) and very often a small backspace and L-shaped enter. You've got mediocre boards like most OEM keyboards (e.g. Dell Optiplex keyboards) But there are very good rubber domes, which are usually older models, like the RT7D5JTW Dell Quietkeys and Keytronics.