I don't claim to have huge knowledge of such things yet but my understanding is these days both are well supported.

What it comes down to is how the feel of it is. I always equate Ubuntu to feeling more like Mac OSX whilst Mint feels more like Windows. Mint also has more customisation than Ubuntu I believe.

I tried Kubuntu for a while but settled back to Mint after a while. There are a huge amount of Linux distros and flavours out there to cater to everyone.