If you would have told me that we would be picking between those two for The Thing, I wouldn't have believed you. I tend to think Rogen would make a better Thing, but Segel is the better actor IMO, so I'm not sure. And just to throw in some credibility, Segel is also Jewish (at least partly).
this should be an easier pick for me, as I've never really understood the Rogen hype (other then being Jewish and have an unique/rough voice) and Segel is someone I've used in some of my fan casts in the past (but did so knowing it was a very unconventional suggestion) at the same time that's what I liked about it, it's not your traditional stereotypical "fancast" pick (kinda funny to me that it's currently even rumored/ trending right now)
neither of them have the traditional look for Ben... but, I think people focus to much on the human Ben look, which will probably be a pretty small part of the role (few shots in an origin story/ or flashbacks) if anything, I'd probably have to give Segel bonus points just on height alone in those regards
(although, the tough guy/Jock persona I feel is a bit dated, anyway, and they might change that up)
he (Jason) is someone {especially with people "bro shipping" him with Krasinski) would at least be around the same height (if not slightly taller) then Krasinski, for their human scenes together (where Krasinski would over tower Seth)
tho, speaking of which, I can also remember a time when Krasinski's looks were considered too "unconventional" for a superhero/leading man type role, so, people eventually got over that
but I'm getting off track here... so what it comes down to is acting and personality
which again {of the 2, not that they are the only two options) I got to give it to Segel