i use a kawai k3, which has fairly good velocity but the best thing about it is the pressure sensitivity, which i haven't seen on any other cheap midi synths. the weighting of the keys is really strange though, kind of mushy (probably the pressure sensitivity somehow) and tiresome to play after a while. the action isn't quick enough sometimes, so when i want to get something in that doesn't feel tight enough on the k3 i use an akai x7000. both of them were around $100, the k3 isn't splittable but i think the x7000 is.
the sound of the k3 is phenomenal for the price, though, i can't recommend it highly enough as an alternative to the similar roland analog line (Alpha Juno, JX-3P). all the options you'd want out of a basic 6-voice polysynth with a sound that, thanks to the digital waveforms combined with the excellent filters, can do just about anything. massive digital sine bass or warm strings and big mono portamento leads. beats every similar digital-analog model for my money, i'd get rid of my mks-50 before losing that thing. the x7000 is one of the real early akai samplers, 12 bits and really fucking weird to work with, plus it uses those quickdisks so i wouldnt recommend getting it unless you found one with the same battery back-up mod mine has. it'll work without a bootdisk though, so if you find someone giving one away and you need a controller it's worth taking home. non-resonant filter, with the ability to store either 6 or 16 samples in memory. really bright samples for some reason, probably because it uses a seriously fucked up method of data compression, some kind of inverse ratio of sample length versus frequency range. the longer the sample, the worse it sounds, up to 8 seconds at 1khz or something like that. keyboard action's nice though, feels like my polysix used to but with a bit more spring