Personally, I think they should just combine the two minyanim. Assuming that's not an option...
Version One: The other minyan has a trend of having 9 men even after the rabbi makes a request. He would be the 9th man.
Version Two: The rabbi says he most likely will get a 10th person, but is not completely certain.
Version Three: Same as above, but the rabbi did not say anything either way. However, he states that after 15 minutes of waiting, at least 1 person who is present will leave. So, even if he and another person appears late, it won't matter, as there would be just 9, anyway.
Version One: Since the other minyan most likely will not have 10 men even if he joins, what are they gaining by having him come? He might as well stay in his own minyan, where he's near assured of having 10, than to switch minyanim, where he's near assured of not having 10. We know how important a minyan is from sources such as this question, or this one regarding how far out of your way must you go to attend a minyan.
Version Two: Even with the Rav's assurance that he most likely will get a 10th person, I still think it's better to stay put. After all, the rabbi is only most likely sure, but not completely certain. The current minyan still has a higher chance of attracting a minyan.
Version Three: First of all, there may very well be a lifnei iveir issue, as the rabbi is trying to get the guy to daven in a minyan that may or may not get a full 10, as opposed to his current minyan which still has a higher chance of getting 10.
But we're here to focus on what the guy should do right, not what the rabbi did wrong. In this version, the guy has no idea whether the other minyan usually gets a full 10, though were he to go there for a few days he would quickly realize his current minyan gets more people. But even at this point in time, the rabbi's telling him something crucial: often enough to make it noteworthy, the minyan dissolves after 15 minutes of not having a minyan. The fact that the rabbi has to mention it should trigger warning bells in the guy's mind.
If you want to ignore that fact, explaining it away by saying that the rabbi's just telling him that on the few occasions when there's no minyan for 15 minutes it dissolves, then he has no way of knowing which minyan is present more often without actually going to that minyan. Thus, that crosses this option off the list for being inapplicable.
I don't think it matters if he was able to return to the first minyan after those 15 minutes. Assuming the minyan begins davening and just stops after 15 minutes, he very well might break his kavanah (see next section) due to traveling. He might as well stay in the now-empty room and daven b'yichidus with kavanah - or, better yet, stay and daven b'tzibbur in the old minyan. Of course, as before, he had no way of knowing this would happen.
He finds the minyan he regularly attends more accommodating to his davening pace. The other minyan davens too fast for him.
In terms of answering to Kaddish and Kedushah, this doesn't make a difference. Not everyone has to answer. In terms of Kavanah, though, it may well be worth it to stay in the minyan that fits his davening pace, since he'll be better able to focus when he's not saying one thing and the minyan's saying something else. (Source for importance of kavanah.)
The minyan he regularly davens in is his Nusach (say Sefard); the other is not his Nusach. / The other minyan is a different "version" of his own Nusach. For example, he davens Persian Edot Hamizrach in his regular minyan, while the other minyan davens Morrocan Edot Hamizrach.
Again, I don't think this makes a difference. As long as he's not Chazzan, he can daven whatever nusach he wants. (See here and here.)
Is makom kavu'a (regular davening place) a factor in making any choice? If he attends the other minyan, he will not be davening in his makom kavu'a.
This one I can't pasken on. I know of the importance of makom kavu'a, but I don't know if it's absolutely critical to daven in your makom kavu'a, or if in a sha'as dechak you're allowed to daven somewhere else.
Now, let's go through the points one by one and see if things would be different if any individual point were false.
Let's say that the other minyan had the same rate of success as his own of getting a minyan, were he to switch. On that alone, I'm still not sure that he can switch. After all, although he's enabling the new minyan to daven b'tzibbur, he's also preventing his old minyan from davening b'tzibbur.
Let's say that the other minyan fit his davening pace. (Note that my answer to this, even disregarding the various versions added in by the OP, is different than what I said before.) According to Versions One and Two, his old minyan still has a higher rate of davening with a minyan, and, as we mentioned above, he doesn't actually need to answer anything as long as someone else is. Thus, I think it would be better to daven in the back and allow for the Shechinah to rest on the minyan than it would be to daven alone and prevent both minyanim from davening with a minyan. I think it's worth it to lose his kavanah to enable nine others to daven.
On the other hand (to quote What If?, "however..."), we also know (Gemara, Kiddushin 64b) that personal chiyuvim take precedence over chiyuvim of others that are fulfilled through you. Thus, perhaps you should daven in the other minyan, since you're more likely to be able to daven with kavanah, assuming that minyan has the same rate of success of getting 10 than the current minyan.
TL;DR: it would seem that in all scenarios he's better off where he is, except maybe in a case where the two minyanim are equally as likely to get a minyan.
Reflecting on that original comment that they should just combine the two minyanim... They say that the minute the shul with 10 guys got an 11th, they were never able to get another minyan. Things would probably be even worse if they merge the two minyanim....
