The cost for a fiance visa or a spousal visa is the same (about $530 at current exchange rates.) The difference is that the fiance then has to apply for Further Leave to Remain which is £150 (or £250 to do it in person). Of course you are spending the extra money to have your fiance fly to the US for the wedding, so depending on airline prices, you probably break even.
One advantage of the spousal visa is that the financial burden of proof is lower, as you are able to work right away. With the fiance visa, the sponsor has to prove he/she can support you until you are married and legal to work. Also, the waiting period to marry in the UK is longer than in most states in the US (about 3 weeks... see the visa faq thread for my experience) so that's nearly a month you can't work, even if you apply for FLR in person as soon as you have the marriage certificate.
We got registered on Jan 31, just before the new law takes place, which is how I'll be able to marry as soon as I arrive, but we very nearly got into trouble. The registrar told us that by registering before the law change on Feb 1, I wouldn't need a fiance visa, and I could just come in on a visitor's visa, get married, and then apply for FLR while in the UK. I didn't think that sounded right, but I had to do a lot of digging online before I could prove it. (And if I'd known earlier, I'd have brought all of his documents with me, and saved the hassle of having to wait for them to appear in the mail! But that's behind me now. My application will be at the consulate this morning, and hopefully will be approved very quickly!)