If I'm attacking an idea by attempting to compose and arrange the entire thing first I'll just use some simple VSTs like a Nexus piano or even FL Keys and just something like BooBass for the basslines.

Normally I end up starting sound design earlier than I intended but sometimes I try to get the entire composition and arrangement done first. I'd avoid using straight midi just because it's really no effort to throw a vst in front of it and make it sound a little bit better.

Like you said, depends on the track!

A lot of my basic drumwork is often done in MIDI with Maschine, but there are plenty of times I'll just drag the sample from my library into the arrangement and just manually build it out. When I'm writing musical ideas I'll usually start by opening a piano sampler and just noodle around on my keyboard/draw in some notes, but I'll also start chopping up some loops or samples in my library and see how I can create a melody by pitching and reversing and all that fun stuff.

That being said, a great workflow to develop is bouncing and resampling so you can jump back and forth between audio and MIDI. Make your basic idea in MIDI, then bounce it all to audio so you can do fine tuned editing, then adding more MIDI to add more polish and detail, bouncing that, etc. Or design a sound, record a few notes, start chopping and messing with them and processing them, bounce that out to a new file and put that into a sampler so you can play a whole new musical idea with it. Or the classic "synths in MIDI, FX and drums in audio" split.

You want to know how to map the MIDI controllers to the knobs on your keyboard ? In this case check your MIDI controller documentation.

Or you want to know how to assign a MIDI controller to an effect parameter in ArKaos ? In this case read the documentation.

The more powerful your computer is, the better you can get the sounds you want to play with before you can record any thing.

That being said, I tend to get my sound first before i can get the music. To me, getting the right feeling in the layer i wanted to add was always the toughest part than actually writing down notes.

I still think it's the toughest part in electronic music because it has just as much as an emotional impact as the notes that are played.

If  i can get a sound that fits with the song, then for some reason the notes i play just come out. But i have to be careful because, some times the notes get weird the larger my note range spans. So it's a balance of understanding your sound and how far it can take you with your music and what purpose of that sound as it resides in your song.

I've always used MIDI simply because it's more convenient, and when I want an organic feel I use grooves. But lately I've been having the idea of only using audio files from stock sounds on my casio keyboard with my very bad playing just for fun, so I may start using audio files more.