FL Studio is primarily a pattern-based sequencer. You build pieces and arrange those pieces to make a song. Other pattern based sequencers include Live and Maschine.
Pro Tools is primarily a linear sequencer. The song is one large sequence of notes/audio. Other linear sequencers include Cubase, Logic, and REAPER.
Although there is some feature overlap, the paradigm is totally different:
- Pro Tools and other linear sequencers aim to strike a good balance between composition and engineering workflow. If you're doing an equal amount of commercial beatmaking/songwriting, mixing, recording, and mastering for other people, a linear sequencer is designed to offer a solid workflow for each of these processes and should work out well for you.
- FL and other pattern-based sequencers, on the other hand, are designed to ease the composition process at the cost of hindering the engineering workflow a bit. If you're primarily making beats and only mixing/mastering your own instrumental tracks, a pattern-based sequencer makes this process easier for some people.
Ki
Salem Beats
EDIT:
P.S., here's an intelligent discussion regarding pattern-vs-linear sequencers on KVR:
KVR: Are You using a 'Pattern' based DAW ?