personally, i can't tell the difference enough to say one is better than the other. i have had a dedicated MIDI controller before and switched from that to a Korg Triton which currently serves as my main MIDI controller and keyboard. the main thing that I like about having the Triton is, as you mentioned, having the option of using the onboard sounds to practice or figure out songs. With today's technology, a lot of pianos with onboard sounds often support MIDI out which is pretty cool for those of us who are looking to have a self-sufficient approach to producing music. in either case, it all depends on what you want to pursue to make your work output much mroe efficient. however, i would suggest that you make sure your MIDI controller or workstation has touch sensitive or velocity sensitive keys to make sure you MIDI programming is translated properly to your DAW environment.
on a similar note, piano workstations such as Roland's Fantom series have gone so far as to add MPC style triggers on its chassis to make purchasing all-in-one workstations a lot more lucrative for home-studio enthusiasts.
