While I don't personally use either lathe in my shop, I think one comment is worth making based on reputation. While I have worked with Rikons, and the ones I have used were fine, there is no denying Powermatic has an EXCELLENT reputation. I use a Oneway 1640, but the Powermatic is right up there in quality, and some would argue a better value for the money (not me, of course!)
You simply cannot go wrong with a proven winner like the Powermatic - it has been around a long time, and it is not likely to go anywhere soon.
"Keep those shavings flying!"