Go with the PMR because overall it is a better marker hands down.
It's advantages over the Ion are few but fairly important as far as I'm concerned, because it has the good bolt out back system, which makes cleaning and maintenance a snap, but it also has a better feed neck then the ion, although admittadly that isn't that good. Also it comes with a better stock barrel, lighter overall weight and the stock board is better then the ion's as it's easier to adjust and comes with debounce, although it's under the name of trigger sensitivity for some reason! And lastly the stock ASA is far better the the Ion's as well, by a long way!
The only real problems with the PMR are the Bolt End Cap because it's made from plastic and is likely to strip requiring you to buy an Aluminium one, the plastic feed neck because if you are not careful with it, you might end up cracking it and finally the trigger. Although the trigger is too bad, it may not be to most people's liking. It's not likely to break unless you're especially vigourous with it but some people just prefer a different feel, really it's a matter of opinion.
But overall, the PMR is still better then an Ion because the Ion requires you to upgrade virtually everything to get a decent marker out of it and trust me I own both of them!