As per Full Circle Magazine.
Similarities
1. They share alot software which means any software you have on ubuntu will most likely work on Mint
2. They both use Gnome.
3. They both offer the other Window environments as well.
Differences
1. Mint only supports 32 bit architecture while Ubuntu supports 64 bit.
2. Codeecs come installed in Mint by default while you have to install them in Ubuntu.
3. Some of the key features of Mint is its set of desktop applications to simplify the users experience. Examples are Mint Disk , MintWiFi , MintMenu and MintDesktop.
The conclusion in the magazine:
if you want an effortless distribution that handles multimedia and wireless out of box then Mint is the right distro for you. but if you need 64 bit or want a huge community support then Ubuntu is the better of the two.
Source : Full Circle Magazine Edition 11 Downloadable as PDF (5.1MB) @
http://fullcirclemagazine.org/issue-11/