Whats better, a rack synth with a cheap midi keyboard or one with a
keyboard built in? specifically novation ks rack or ks4?

cheers

Depends on what you want.
Generally, its good to have it all in the keyboard, if you want to take
your keyboard and play live some place.

On the other hand you can get very good used keyboards all over the
place, so a rack can save you a lot of money over the keyboard version.

You can drag the rack, and the keyboard to other places to play live,
but its just a bit more messy, and one more thing to pack around, but
if you can put up with that, then it can be done.  Just hook a midi
cable directly from keyboard to rack midi input.

Just try to get a not too cheap keyboard, one that has good action, and
at least velocity and possibly aftertouch.  You can find them in hock
shops for around $100-$200, keyboards that at one time sold for more
than $500