If my preferred MOS isn't available and does not have many slots, should I wait for an opening or should I settle for another MOS?

Here is the thing. When you go to MEPS, if the MOS you want is available, and you qualify, then take it. If it is not available, then you will be shown what is available based on what you qualify for. If any of those raise your interest, then pick one of them. If none do, and 15W is what your heart is set on, then hold out until it is available. Only you can make that call.

If that’s what you really want and you’re qualified for it, don’t take no for an answer. If it’s not available now, wait until it is.

Going to MEPS, you are in a "partial drivers seat," by that if that is what you want, then don't take no for an answer, if it means waiting a while, then wait for it. Because that recruiter has a mission to make, as does the person who is showing you jobs at the MEPS station, they don't care if are a 15W, or 13B (Cannon Crewman). They need to fill slots, If they can get you to fill a …"less than desirable MOS," so much the better for them. So keep that in mind, Now if you want to be airborne, air assault, danger ranger, and 15W doesn't do that, well don't think they set you for both of those. You can go one road or the other, choose wisely.