I have both a Flipside 400AW and a Photo Sport 200AW.
While the Photo Sport is great for keeping light with a body and two lenses plus various hiking goods (snacks/lunch, tools, water) and some gadgets like GPS, it's not the best suited for holding a tripod bigger than those small travel pods (mefoto-ish).
The Flipside 400AW is a bit bigger allowing for more camera gear and less hiking goods/gadgets (unless you place them in the camera compartment). A big positive is the dedicated tripod holder on the back of the pack. The rear access isn't too restricting as I've always found I'll take a break to put the bag down and pull any gear out while I have a snack. Although, you can always attach the hip straps, slip off the shoulder straps and spin the bag around you for easy opening on the go.
Overall, I'd say it's a decision between tripod compatibility and camera gear capacity (Flipside) versus the lighter weight and convenience of smaller capacity and side opening (Photo Sport). I ended up wanting both so I picked up a F-Stop Loka (tripod straps, lots of camera gear space, lots of hiking gear space, but no side opening).
Shameless plug: I'm selling the Flipside 400AW and Photo Sport 200AW now that I have the Loka. Let me know if you're interested.