I am a game developer.I want to make music for my games with fl studio. Should I use a Midi keyboard or a digital piano.
If the keyboard to piano roll feature in FL Studio doesn't quite do it for you a midi keyboard will - digital pianos are made to be played standalone though some can be connected to the computer.
Automation is a big part of computer music and a midi controller that also features knobs can prove really useful along the road. 
Faders are nice as well for controlling mixer levels, yet this is easly done with the mouse. Drum pads are also cool for making beats or choping and re-sampling stuff
These are all optional features and usualy cost more to have them all in one package and as a visual representation of the usefulness of feature, my opinion is keyboard > knobs > transport buttons > drum pads > faders
You should use the piano roll. FL Studio's is really good and a midi keyboard is not necessary. If you have a higher budget though, and really prefer it, you can buy one, but I really don't think it is necessary.
All modern digital pianos have MIDI included.
If the goal is to have the most realistic keys I would go with this advanced MIDI keyboard using Keyscape or Ravenscroft samplings.
