You don't get the choice, all payouts are in US dollars. The easiest way to recieve payment is via PayPal. It will arrive from Zazzle in US dollars, with any currency transactions converted to US dollars at the exchange rate on the day of payout.

From there you can link your PayPal a/c to the bank account of your choice, and with the click of a button transfer your earnings to that account (takes about 3 days). No fees are incurred by using PayPal with Zazzle, either for receiving or transfering the money.

***EDIT to add, I've just re-read your post and realising that you may mean you can't decide whether to if it paid to your US or UK bank account (?)...Hmmmm well if it was me, I'd just pay it to whichever one I'd be most likely to use it, unless you are earning thousands a month the exchange isn't going to make enough difference to worry about really...just my opinion