Thanks for the Proof of Stake reference, I was not familiar with that system but am reading about it now and it's very interesting. This site [1] seems to indicate a rather unfavorable transaction cost for Bitcoins, around 7%. However, I'm not sure whether their estimation of 650 watts per gigahash* is accurate (that warrants some more research), so I don't know whether to trust that figure or not. If it is accurate, it is not favorable when compared to Visa.
[1] https://blockchain.info/stats
* I assume they meant watts per gigahash/second.
[EDIT] It seems like the recent ASIC miners on the market are vastly more efficient (>100x) than blockchain.info's estimates [2] (which probably include CPU/GPU miners since ASICs are relatively new). Without knowing the makeup of the Bitcoin network (i.e. what portion is CPU/GPU/ASIC etc), though, it seems like any efficiency estimate will be pretty inaccurate.