What's the difference? Will POR wick into the pinchwelds? For some reason, all the cars we've done in the past twenty years that were sandblasted and epoxied have not rotten out. If there is substantial corrosion between the pinchwelds that the car will rot in a short time after proper prep and epoxy, the car was a POS from the start.
Rust doesn't creep under properly applied POR15. Sometimes it's not possible to get all the rust and I'd be concerned about it creeping out again particularly on outside seams like the rocker panels, tail panel, and cowl area.
Really though, with a lot of the cars we're talking about, they get stored inside and driven sparingly so really there is not much of a concern either way. It's just that some people don't have access to sandblasting equipment nor do they have a place to do it anyway so other means need to be employed. POR15 is a viable alternative. When properly applied to a car that's stored inside and driven sparingly it will outlive the owner most likely anyway.
Some people are working on a "POS" because that's that they have to work with. Plus, some cars, even if they are a "POS" by your standards, are worth rebuilding - such as an SS or perhaps a car that's been in the family since new, etc.
I'm not slagging on your way of doing things, it's just that some people don't have the nice warm cozy shop with tons of room, a rotisserie, an area for painting and an area for blasting, and a compressor the size of Texas with which to restore their car. Products like POR15 and RE give the driveway and garage restorers who don't have a bodyshop attached to their house nor the money to have someone restore their car for them a few other options that make the restore a little less costly.
To the OP, unless you CAN get the underside sandblasted and perfectly clean of ALL rust, I would use POR15 first and then put epoxy over the top while it's still a little tacky. I did that on the rear wheel area of my 69 4 years ago and did some test removal in places when I was getting the underside ready for the final coating last month, and it was pretty tough to get off. I'm not the least bit worried that it will fail anytime soon. Just follow the directions for POR15 and make sure you have a nice rough and CLEAN surface and it will never come off on its own. POR likes rough surfaces. I personally would not use it over any other paint, only bare metal sanded well with 60 or 80 grit, rusted surfaces that have been wire wheeled and sanded with 60 - 80 grit, or freshly sandblasted metal (although this is the perfect surface for epoxy). I do trust the POR on formerly rusty surfaces that have been sandblasted more than epoxy because unless you get ALL the rust off it will come back under the epoxy.
It took me several years to finish up the underside of the car. I replaced main floor in 2006 and the trunk floor in 2007 and that's when I cleaned up and painted the area between the trunk and the main floor. I was pretty confident I had the wheel houses clean (they were half new anyway as I sectioned in the lower part) so I used just plain epoxy on those. I had POR and epoxy on the area between the tunk and floor, ecoat on the main floor, and epoxy on the toe kicks. It sat inside for about 3 years after this as I was working on other things. Then this winter I cleaned up the ecoat and bare metal areas left from welding the floors and patching the inner rockers, scuffed the crap out of everything, seam sealed, shot RE over the main floor and inner rockers and then applied Herculiner over the entire underside. It looks great and I'm not expecting any problems.
For areas that you will see, like the firewall or the forward part of the frame, I'd use Rust Encapsulator rather than POR.
