Let me try and weigh in:
2) Ease of Access for New Users
Discourse has a pretty good onboarding procedure, it allows people to join using not only a username / password but also log in via oauth providers like Facebook, Google, etc. I don't know if the current versions of vB or Xf (is that the official abbreviation? Thank fuck because xenforo is a shit name lololo) have any of that.
3) Ease of Data Migration
I expect both vB 5 and Xf to be very easy in that regard; vB will have a standardized upgrade / migration script, Xf probably will too. Discourse has a vBulletin importer too (two even), but when I tried it it took three days for the initial import, and then it needed to do a million background tasks - I think that was shit like send returning members a message with information, recalculating stats, etc. I ran that on a local development environment and it kept freezing up.
4) Theme Capabilities
That's something I never liked about vB, while it does offer all the customization options, it mainly does so via their own web interface, which is a poor user experience. I don't know if Discourse has a good alternative, there is theme support somewhere but I haven't found any good guides about it. Ideally there'd be a development mode with simple code / css editing and live reload. Might be possible if you have a dev instance of the software. I don't know about Xf but I can imagine they've basically done the same that vB did.
5) Mobile-View Compatibility
vB and Xf will probably come with mobile skins out of the box. I know vB 5 was a lot heavier per page than vB 3 though, iirc each page was at least 2 times as heavy - and that wasn't counting images. Discourse comes with a responsive design and should work fine on mobile by default (although it's not great IMO), and there's
an official app which (probably) hooks into the API (Discourse has a strict back- and front-end separation which allows that).
6) Moderation & Staff Learning Curve (vB > Xf > Ds)
I know Discourse was made by the same guy / guys behind Stack Overflow, and managing spam and abuse and setting up a community without abuse has been one of the major challenges they had to overcome. With that in mind though, moderation didn't seem as heavily emphasized or developed as vB; it probably doesn't need to either. What I recall is that users can flag posts, moderators will get notifications, and can take actions ranging from warnings to shadow banning.
here is an article from the founder of both SO and Discourse, he's also very active on the Discourse meta and is probably pulling the project.
7) Ease of Back-End Troubleshooting
Urgh. vB / Xf will be easy enough to reinstall in case of errors, in the case of Discourse I just don't know. Discourse does seem to have a decent backup strategy though, and backing up / restoring from my dev machine to the test site was easy and fast - I'd argue it was easier / faster than vB was.
8) Security Concerns
vB 3 is shit, that's for sure, and it's amazing we haven't been hacked yet; I think the fact we removed visible version information from the bottom of the page helped in that regard. Mind you we probably have been hacked already, possibly through Wordpress but at this point it could be anything; the hacker just stole the database instead of defacing the site (like what happened with FFOF). Xf, I dunno, I like to think the PHP stack is safe enough nowadays. Discourse I dunno either, but I get the feeling there's much more development power and attention to e.g. security than there is for vB and Xf combined.
9) Shelf-Life / Sustainability
As I mentioned elsewhere, vB 5 seems to not get much updates anymore. I don't know about Xf, but given it's closed source I also get the impression that it's only going to get maintained for as long as the creators can be fucked / get paid. Discourse is heavily invested in atm, and they get
hundreds of contributions a month, releasing at least once a month. I saw they have just released the 2.0 version, too.
On that, I noticed Xf also has a 2.0 version now. Their announcements page seems to be updated frequently enough.
10) Cost: Monetary & Time
vB 5 is $250, possibly $209 as an upgrade (we have an existing vB 5 license actually); 'patch' updates aren't listed on their pricing page. Upgrading would be a matter of hours, plus whatever time it takes to make the visuals work but I dunno if I can be fucked with that.
Discourse is either free or $300 a month or more, depending
Xf is $140 initial purchase, $40 per year after that for upgrades. Their website looks dated and shit btw, jesus christ how are they still in business.