Among other reasons, I went SWO(N) over Subs because I didn't want to end up on a boomer steaming in 5 knot circles for three months at a time.
There is always the possibility of being voluntold to go to a boomer. However, chances are that if you want fast attack, you're going to get it. The number of SSNs are greater than the number of SSBNs. On top of that, there are always going to be married people or priors who have "been there and done that" who want the consistent 3 month out/9 month in schedule.
The big thing that most guys are going to be competing for is SSGNs, which have the spaciousness of a SSBN, but travel to foreign ports. Myself? I'm hoping to score a Virginia class boat, but any fast attack will do. My class is on the young side due to the academy and NROTC graduating, though, so we'll see what happens.
The short of what SSNs do: go places we don't belong to perform missions that we don't want people to know about.
The CO and XO of NNPTC managed to do their entire careers on SSNs.