Wow, that is a tough question. "Long term value"... hmmm... Both guys are 28 so no edge there. Both very talented. Both #1 WRs. Fitz is more reliable but VJax has the better offense around him....
I think you'd get a higher price for Fitz because of his more stud-like status. Don't think you'll get much for Jackson. And VJax has to break out eventually, right?
VJax by a nose.