Steve is right. I haven't seen any difference in selection between Marines and Navy SNFOs at VT-86. It's all about the NSS. Oddly enough, it's the Chair Force guys who get the near guarantee of F-15s (assuming you don't screw up).
Hornetframer- just to piggyback on what's alreaday been said, it's actually reasonably predictable compared to the pilot pipeline. If your NSS is good enough, you'll probably get what you want. If it's on the rocks, you might get shipped off on any of the other pipelines somewhere along the road.
Here's the catch- selection for Super Bugs vs Prowlers (S-3s are still open now, but won't be by the time you get there) is based solely on what you do in your first 6 weeks in advanced. Your API, Primary, and Intermediate grades don't matter at that point. Only the 5 sims and 9 flights in Advanced. Down one, and it's unlikely you have enough room to recover.
It's all about busting your butt for the grades.
Also, would having a background in Hornet maintenance (airframes) help at all?
Nope.