Some general, rather than technical, comments:
1. MIDI export defines my work style. I often create the framework for songs in BIAB, then port them into Propellerhead Reason, which is, among other things, a high-quality MIDI-based soft synth. From there I revoice, expand, and massage at will.
2. Jford’s remark about “different synths” responding differently to velocity is actually a patch-dependent effect called “zoning.” It is caused by mapping different samples (digitally recorded sounds) to velocity values. A simple patch might have a clean electric piano sound from 1-80 and a distorted sound from 81-128 to simulate overloading an instrument amplifier at higher volume.
Of course, it can be much more complicated; I have a book on the subject. It is not germane to the discussion, but it’s pretty interesting!
Richard