Should I use vBulletin 3.6.12?
 

Since there are a lot more styles and modifications for 3.6.12, and vbulletin.org is using it. Would it be a good idea for me to use it?

You can if you wish, but I would suggest getting the newest version which is 3.8 ATM ;)

Any security differences?

If your doing a new install I'd go with 3.8

If your currently using an older version, nd dont need features in 3.8 then using 3.6.12 won't be of any harm. There shouldn't be any security differences.

PS most the styles upgrade easily from 3.6.12 to 3.8

Well, just like 3.0 is going to be archived soon, 3.5 and 3.6 will follow not so long after.
If that's the latest version of the 3.6 series, then I would assume it's as secure as 3.8

I was just wondering because I see many major sites like digitalpoint, vbulletin.org, etc using 3.6.

Is there a good SEO hack for 3.8 yet?

Digital Point doesn't update due to the sheer size of their Database and the owner is pretty much MIA there.

vB.org doesn't update due to extensive custom modifications on the back end, and they don't have a need for the features here.

There is tFSEO and vBSEO for 3.8 ready to go.

So basically the older versions are less resource intensive because of the less features?

It could be looked at that way, but the new features aren't to server intensive by default unless they've been modified or aren't optimized properly :up:

If you don't use those features on the new version, they will be the same. :) There might even be some optimisations done in the newer versions.