Between the 100L and 135L (I have both), I'd choose the 135L for portraits - the extra stop means more OOF blur and the bokeh is better. But...portraits in what setting? Indoors, 135mm is a bit on the long side, good for tight portraits. Outdoors, it's great (you may need a 3-stop ND to shoot wide open). Indoors, I prefer the 85L but that's over your budget - the 85/1.8 delivers very good IQ, IMO it's one of the best values in the Canon lineup in terms of IQ for cost. Lots of good things are said about the Sigma 85/1.4, but I have no experience with that lens.
I'd only get the 100L if macro was the primary intent, secondarily to use for portraits.