Your kit lens covers both of the usual suspects' focal lengths. Either (or even both!) look back at your previous work and see which length you use most for the type of thing that most takes yor interest or tape the zoom ring at both lengths and see which you prefer. Once you know which focal length you're after, you need to decide on the aperture you want. IMO (and I may well get shot down for expressing MY opinion!), ultra shallow depths of field don't make for pleasing portraits - I would far prefer to see the eyes and the nose in focus rather than just a mm or so of eye. Therefore, IMO, you'll be better off with an f/1.8 (or even slower) lens than you will be with an f/1.4 (or even f/1.2).
You could also consider one of the slightly slower zooms - plenty of f/2.8s available to cover the focal length range. I sometimes use my 24-70 f/2.8 Sigma for people shots (don't really do portraits as such) - plenty fast enough for my needs and enough DoF to keep most features acceptably sharp when focussed on the eyes. Far more versatile than a prime and I often find myself in situations where footzoom (moving to alter the field of view) isn't an option (can't walk through walls and I'm Nod, not god so can't walkon water...) 24mm isn't too flattering for face portraits (but can be used for slightly comedic affect!) but from 30mm upwards, it's fine.
