It's all about "the look" AND how you'll use the bike. Gonna do a lot of long distance riding, especially 2 up? I'd buy the Ultra every time. Or do you mostly do solo day rides with maybe a couple multi-day trips per year? Then maybe the custom is right for you. It's all about YOUR priorities. I use my RGC mostly for day rides with maybe 3-4 multi-day rides per year. I liked the overall stance and the look of the RGC, not to mention the savings at purchase. I put a chopped tail pack on it and it does everything I want it to do in all circumstances. For others, the chopped TP isn't big enough, they do a lot more 2 up riding perhaps (gotta have room for the 6 pairs of the wife's shoes, etc.!) and they like the fact that they can buy it set up pretty darned well for touring right out of the box. I have to say that when/if I retire someday, I'll much more likely gravitate towards the Ultra as I'll have a lot more time for long trips. At that point it's all about function over form. Right now, I'm about 50/50 with the form vs function issue.
You'll have to do some soul searching about how you'll use your bike and which one fits those needs best. Then go back and ask yourself which version grabs you by the throat visually and makes you want to lay down your hard earned cash for it. Hopefully those two are the same bike for you. If so, you're decision is made.