I have the PM64a and absolutely love it. It'd be an upgrade to what you have currently, for sure. But it's not miles above, it's a damb fine saw and really there's only one path up from here - cabinet or bigger.
If that extra inch is important to you (think of all the standard sized blades you could have!!!), upgrading's a certainty. If going cabinet is easily a few years away, then I'd say get the PM. You could probably use your PALS from the delta on the PM (if you reuse the PM's bolts).
As I write this, it occurs to me ... the PM 64a brand new is around $700 ... I've seen used Unisaws for around that on Craigslist. If the deal you're getting is say in the $400 range and the saw's basically new looking you're probably safe. But if we're talking somewhere under $300, I'd seriously check out the bearings and look for any damage before taking the plunge. If all is well, I'd jump on it