Spend the extra $100, it is well worth it, I love my adjustable backrest. After riding for a while I too like to lean back further or maybe sit more upright couldn't do it with the fixed rest. It' s worth it!
Adjustable has got to be the way to go with back issues. For me, I'm like Hobbit, feels best to change position every once in awhile, no matter how slightly.
Bottom line - you're riding a Harley for Pete's sake, let's not get cheap now and worry about a hundred bucks!