A couple of quick points for your post:
-to get either a spousal or fiance visa, she'll have to apply from the US
-in order for you to marry in the UK, she'll have to apply for the fiance visa, otherwise, you have to marry in the US and get a spousal visa
-people sometimes find it cheaper and easier to go the spousal visa route instead of fiance; you're only technically paying for the one visa (spousal) instead of two (fiance, then FLR once she's in the UK). Plus, on a spousal visa, she can work from the get-go; on a fiance visa, she can only work once she gets her FLR. It just depends on each individual situation, methinks.
-application times and fees between the two visas are fairly similar--if it's fairly straightforward, and the consulate isn't too bogged down, and you use a courier, it can have a turnaround of a couple days
-as for how much money to save, that's difficult to say; loads of things can change in the next couple of years, but generally the ECO looks for the household to have 95 GBP left over per week after bills. The important thing is that the ECOs are looking to see that she won't need public funds while she's there on a spousal visa, and that you two as a couple will be able to support yourself without public money (you can claim any benefits or credits you would be able to get on your own, but you can't claim any extra on her behalf).
It's good to be informed, but a lot is going to change in the next few years, so just be aware of that. Whatever we tell you now may very well (and prolly will) be different.
x-posted with everyone else