I think that if users of a particular device want to have a watering hole then it would be better to have them within the forum for that device, because then it allows new members and owners of that device to join in. If the watering hole is in the Lounge or a sub forum of the lounge, new members would not know of its existence, and they would therefore not be encouraged to explore outside of their device forum anyway.
However, I'm only here because of EarlyMon's link from a new S2 users' off-topic thread recently moved to the Lounge, where the guys wanted to remain inside their "All Things Root" subforum, and he invited them here to join the discussion. Now because this was a short thread with not too many posts, I thought I'd take a look at it, and became quite interested in their plight. They seem like a nice bunch of guys who enjoy hanging out together, with a real sense of mutual appreciation which was actually touching.
The thing is, although there is currently a "Moved" post within the original forum pointing to the new location in the Lounge, this will soon disappear and new users will not know about its ever existing. Also, a problem I see with moving off topic watering holes to the Lounge or a sub-forum of the Lounge is that the frequenters of those watering holes will no longer be reading their original device forums because they will be looking in the Lounge for the latest post to the thread. If the thread was within its original device forum, those members would also see other threads in the forum and possibly reply to them, or read some device-specific news they would not otherwise hear of. In essence, they would be less isolated from their larger community of same-device users. With sequestered or remote watering hole threads, users with device experience are being removed from the very forums in which they would be an invaluable resource to other users in need of help - particularly in the case of the "All Things Root" crew mentioned earlier.
If all the S2 rooting veterans are off in the Lounge or Social Group chatting away, who is going to help some desperate noob who's just sent his S2 into a bootloop?
Now one argument is that I would never have likely known their existence if the thread weren't in the Lounge, but I would probably not have read the thread if it had not been a new one. I have little inclination to try butting into some 1000-post thread frequented by people who have got to know each other and at least have their device in common, and so I don't feel those watering holes being in the Lounge is going to lessen the segregation and clique-making that is inevitable in any community.
So, if you haven't already guessed, my choice would be option 3