I've just done both last week.
I was quite disappointed with Plaza Inn, I think we were there about 1.5 hours, and the majority of characters stayed inside for the majority of time. We were seated outside and it took a LONG time for any of them to come out, let alone any variety. I asked the character handler and I was 'managed' and told the only guarantee is to see Minnie, if you see anyone else, that's luck. I protested there were about 4 characters inside v's only 2 outside and the rotation had been slow, but was pretty much told tough luck.
Considering you're not meant/allowed to approach characters at these dining events as we finished our meal and it was a bit slow, we ended up going inside and grabbing a table near where a couple of characters were, so we were technically seated, and we were able to meet a few more.
Goofy's on the otherhand, had a wider menu choice, (Plaza Inn doesn't have the variety) and the characters were fairly good at rotating. Not only that, I was keen to meet Rafiki. I asked the waitress if he was coming back out as he'd disappeared, she asked the character handler, and she came back to let us know he was having a little break, but would come back and would start with us. Much more impressed with this level of service and accommodation (though that being said Rafiki hadn't been on our side of the restaurant at all, so it wasn't just 'us', but we were handled in a much nicer way than at Plaza Inn).
Both were good for meeting a variety of characters, but if someone had to pick one, I would recommend Goofy's kitchen for the selection, and if you end up at Plaza Inn, ask to be seated inside!