Danks probably has the better upside because he's younger and lefthanded, but I like Diamond better.
Diamond dominated the at the A level but struggled at AA. Yet he still allowed under a hit per inning at AA last year while striking out 68 in 69 IP. The number of walks was concerning, but he has shown better control in the past.
Danks dominated at Clinton and Bakersfield at a younger age than Diamond, but the domination was speckled with 55 bad innings at Stockton and 12 horrible innings at Stockton. Danks was younger upon promotion to Frisco, but it's hard to get excited about his numbers, except in the scope of age relative to league. He allowed a lot of hits, too many homers, and the strikeout rate fell off.
Danks and Diamond are two of the better pitching prospects who have yet to make the full AA transition. As of right now, I think Diamond will be ready for AA and AAA next year, while Danks is still a ways off.