I've owned a couple of Boxsters along with a few 911's.
Boxsters are close to perfect.......IMHO
German build quality and reliability, 2 boots, all the power you could reasonably use in the real world, amazing handling and convertible. What's not to like........
If it goes wrong it will be expensive/very expensive, but that's probably true of all modern cars, but if you get one from a main dealer you get a 2 year warranty on a used car!!!!
I've been up to the porsche experience centre at Silverstone as a guest quite a few times and driven Boxsters on their track, they really come in to their own there, the balance is amazing.
However...........
A mate of mine very recently had exactly the same dilemma, admittedly he was looking at a slightly older Boxster though.
He bought the MX5.......
He got an absolutely ridiculous deal that he couldn't turn down on an ex demo car from a Mazda dealer.
It's nice......but......The Boxster is better, he knows that, but the deal on a much newer car was un-turndown-able.....
Get the Boxster, you'll probably end up getting rid of the plus 8...........