Am I the only one who finds the shift from the big to the little ring on a compact crank (50/34) to be too much of a jump? I tried riding compact for a summer and found myself having to simultaneously upshift in the back each time I downshifted up front to avoid losing too much speed & power--or having to spin too fast--with the shift.
I would advocate for going with the 12-30 cassette if you need more range: it'll even give you a higher high gear than you've got now along with a lower low gear than you'd get with a compact crank and your current cassette (check out Sheldon Brown's bicycle gear calculator here: http://www.sheldonbrown.com/gears/index.html ). You'll be going up 2 teeth at a time on that cassette, but I find that to be a pretty natural jump, and a good tradeoff for that massive range.
And--you'll spend a good deal less on a cassette & a new rear derailleur than you would on the compact crankset & fr. der.
Out of curiousity: where'd you find a 12-30 cassette?