1. Why not have both? The market will decide which one people choose to use most. One might be used more at first, then later as browsers are updated, site owners might later switch to the other. But.... Are they mutually exclusive??
Nope, the hard parts aren't parsing the bitcoin: url string (or the file), and once parsed the same code could do the actual request.
2. What's stopping us? Let's just implement them BOTH immediately. We obviously have the knowledge and expertise to get the job done. Let's just get the job done already.
The hard part: what happens if I click on a pay-using-bitcoin link and I don't happen to have bitcoin running right now?
The browser or OS runs bitcoin and hands it the payment request info.
The bitcoin process checks to see if there is already another bitcoin process running.
Nope. Ok, load the wallet. And then wait until we're caught up fetching the block chain that we missed while we weren't running. Gotta do that because some of the transactions in my wallet might have been spent (if you copied your wallet somewhere), or you might have received payments while bitcoin wasn't running.
And so N minutes after clicking bitcoin is FINALLY ready to send payment. If N is greater than 1, then that really sucks!
I like click-to-pay, and I want it to work; it would work well now for the "I'm running bitcoin and connected to the network 24/7" use case. But I don't think that will be the common use case (most people probably won't bother generating).
So I think we've got to figure out some clever way of making click-to-pay quick-- maybe ask for payment permission and then have bitcoin chug away in the background, popup an alert if there's some problem paying, or just shut itself down after it's caught up with the block chain and has submitted the payment. Or, assuming you have enough coins in your wallet, maybe just throw the transaction onto the network and let peers tell you if you're accidentally double-spending (that makes me very nervous, though). Or...