The more I think about it, the more I like the idea of it as the next single. I mean, release TM to let people know they have the free album; it will attract a lot of older fans (even the casual ones who may not have otherwise got the album) through the commercial, the free album promotion, and to a much lesser extent the [relatively small] amount of radio play it has received thus far.
Then, when the physical release is set to drop, hit the world with the crystal ballroom and promote the hell out of it. TM will have already let the casual fans know you're back, and The Crystal Ballroom will be the best shot at gaining new fans. (I'm not gonna mention the die-hards here, because they would have found out about the album one way or another and got it regardless of single choice.)
It's also a great way to promote the physical deluxe release, which is what needs to be done, because virtually everyone who wants SOI already has the other 11 songs. Like I said, this will consist of casual fans who found it because of the commercial/TM/free album combo, as well as some people who may not have know who U2 was, listened to the album because it was free, and liked it. I mean, what good is promoting TM more going to do? Or California? Or EBW? Sure, they may experience a small splash of radio play that could attract a handful of new listeners each time, but that group is going to be negligible because so many people already have those songs. TCB is both different sounding and not on the free giveaway--it gives people a reason so rush out and buy the album, and it's different sound will have the best shot at attracting new listeners.
Which brings me to my last point. Hit singles these days need DOWNLOADS. So many people already have the other songs... it would EXTREMELY difficult for them to chart because of that. The Crystal Ballroom, on the other hand, would have to be sought after and purchased. I'm not guaranteeing it will be a radio smash, but I genuinely believe it's their best shot. This being the case, having a song played on the radio while simultaneously getting massive downloads is required for a hit single these days.
Clearly, it's the only song that has the potential to do both.