This all depends on what you want to use it for. I have a 12.5" Delta planer I have been using for years (mostly non-guitar work). I also have the 18" Delta sander. The sander is a great tool for guitar work, but it does NOT replace a planer. Moving from a 12" or 12.5" planer to a 13" planer makes essentially no difference in the real world. The two speed option might make some sense if you do a lot of work on highly-figured woods, but in my opinion it isn't worth very much extra money. You won't be putting a full guitar top through it at once anyway ... really just for rough sizing multipiece body blanks, neck blanks, etc. Final work would then be done with planes, sanders, etc.
A 15" or larger planer is a great luxury for sure, but at a significantly higher price point. If you can afford it, go that route. If not, a 12.5" planer is a very good tool to have around. Also keep in mind that the planer is not going to handle very thin stock either ... you won't be doing acoustic tops on most of them for example. That's another good time to pull out the wide drum sander. It's a different tool with different uses. One doesn't replace the other.
Decide what kind of work you really want (and need) to do, then buy your tools appropriately. A small planer won't help much if you only want to do one-piece bodies and tops, but they are great for a lot of things.