This is mostly anecdotal but someone asked me for the numbers today. I figured other people might like them so I’ll promote this to a post instead of a comment. Note that the sample size is fairly low because I made a major error 2 weeks ago and that resulted in my sales declining about 80% over the period (d’oh!).
From a universe of 29 transactions, 13 were Google checkout and 16 were Paypal. (This excludes one additional transaction which would have been Checkout if not for Google denying it for, ostensibly, security reasons.) The presentation of the Checkout and Paypal buttons is fixed and exactly symmetrical unless folks choose to buy directly from lower on the page rather than from the cart. Most (80%) of my transactions come from the cart.
Interestingly, none of my Google checkout customers had their account prior to signing up to do business with me. Back when I accepted primarily Paypal, a rather high percentage of my customers had existing Paypal accounts (its inefficient for me to check exactly how high, but I have quoted 60% at previous times so lets call it 60%). The percentage of Paypal users who have preexisting verified accounts has gone up significantly since I began offering checkout. My interpretation of this data point is that people who already use Paypal are likely to seek it out as an option when given comparable alternatives.
I should note that my cart interface always presents Paypal and Checkout in the same order (Checkout on the left, Paypal on the right). If I had my druthers that order would be randomized for every load of the cart. I suspect many customers are just mashing on any button that they know is in the contextually accurate place for Continue This Transaction Darn It rather than making a conscious decision for or against either provider. Thats fine by me, incidentally, since I hate having customers have to decide things prior to giving me money.
I should note that my demographics are probably vastly more accepting of Paypal than some demographics. Many software developers, for example, hate Paypal like the plague for some reason. I think this viewpoint is rather less prevalent among middle aged ladies, who make up a good deal of my customers. eBay is a big hit with them, and Paypal acceptance among eBay users is extraordinarily high.