If your primary reson for getting either a G30 or G36 is to carry on the ankle as a BUG and must pick between only the 36 and 30, the 36 is the way to go. Less capacity and no compatibility with other Glock 45's, but a lot lighter/slimmer/smaller than a G30.
I had entertained the idea of using a G30 as an ankle carry during the winter months before buying one, but that wasn't the main purpose for it for me, it was more of a 'it would be a nice bonus if I could...' thing. Ater buying it, I quickly determined that it will not work for me as an ankle carry. Just a little too big, and quite a bit heavier when loaded than I thought it'd be. But I still love the G30 because it fits my primary reason for getting it: it conceals extremely well for me to use as a primary carry IWB, and it is also really really FUN to take to the range and is incredibly accurate!
Bottom line, make darn sure whichever firearm you do get, that it *will* meet at least the primary purpose for why you are buying the firearm in the first place. Because if you don't and find out that what you bought doesn't fit the bill, you will never be truly happy with the firearm and will likely trade it in or sell it at a loss. Not that I've ever personally made such a mistake
If you were to ask 'what suggestions would you make as candidates for an ankle BUG', I'd not suggest either the G30 or G36 but rather along the lines of those pocket 380's or pocket 9mm's. Personally I don't much like the 380, but strictly in a BUG role? I like the idea of a pocket 380 or 9mm a lot.