As a former B-52 Radar Nav I can say that I felt I had a bit of a jump on my UPT classmates, but that didn't last long. An F-15E WSO would have a huge advantage in UPT. As a former nav and more senior officer you get points for being the class leader, which helps in your class ranking. Used to be the prior navs would all find themselves in the top third of the class. Don't know how it's been since 16 years ago. I got all my air sickness out of the way (9 hrs at time in the hole will do that to you), and I felt I had better air sense and SA in my first month of flying than I otherwise would have had.
Used to be a wing would get 3 navs picked up every board, then it went down to barely any slots for navs. It's eventually picked back up; the navs I know who are sharp and have a good rep are selected. We've had some who weren't even out of their MDS training and were told they were going to UPT.
Good points made in earlier posts...specifically the money to get some ratings. As for how it would go for someone who went in as a nav and then got a UPT slot? Well, I think you'd be looking at 20 years on AD unless you palace chase (if they brought that back). Think: NAV school, UPT, 10 year ADSC....so at least 12 - 13 years in before you could go airlines (if that's your eventual goal). Like the majority of AF pilots I know today, most are staying in for the security of a retirement check and healthcare. Then they'll consider the majors. That's how it worked for me and I have no regrets. Well, I do have one: If I hadn't gotten picked up for UPT I would have wondered why I wasn't thinking about the Guard/Reserve at the same time in case my active duty UPT plan didn't work out.