Man, I thought this thread was about something totally different...
Anyway, I'll post a vote for bolt-on. First reason, which is likely BS, but I've got this impression that welding to the frame could potentially weaken it. Again, probably hogwash take a look at your frame and all the welds already on it... then again, I'm not a welder so wtf do I know, needless to say I won't be welding anything to my truck.
Second point: what if something happens to my truck, and it's more cost effective to just start over? What if some dumbass slams into me, or I drive off into a creek, or my car catches on fire.. I dunno. But if that happens, I'll be snatching up another locked 80, then will promptly unbolt my front bumper, sliders, TC skid, rear bumper, CO2 mount, internal tire carrier, suspension, roof rack.. whatever else I stuck on the truck, and transfer all of that over to the next one. Let the insurance company total a 1996 Land Cruiser, not a built up truck that I've invested so much time and money into.
And your point above is totally valid too.. O2 sensors you can swap with sliders in place, but what if you did need to mess with the cats, or upgrade to a cat-back system? nice to be able to take the slider out of the picture, if needed. Or what if an even cooler slider becomes available in a couple years, and you want to sell your George's to someone and get something different?
So evaluate for yourself what's most important: strength, removability, portability, ease of install, quality, price... then you'll have your answer. The fact that you're upgrading & wheeling your truck is most important, the rest is just details, right?