Ride both, the major difference is Nightster has a Narrow-Glide front end and the new Custom has a Mid-Glide. They feel very different when you go into a turn. You'll know which one you like better. Everything else on the bike is easily changed or swapped. Front ends are very expensive compared to the other parts on the bike.

 

I've made it 145 miles before on a tank of gas on the N. All highway miles of course but it's capable of getting 60+ mpg.

After 145 miles on a Sporty, you probably want to stop and take a break anyways.

 

I just recently bought a xlc. Love it. I looked at nightsters and almost bought one. But, I didn't like that it had the smaller tank and couldn't ride 2 up without dumping some $$$. I wasn't really looking at sporsters but when I finally saw the one I got, I knew the wait was over. I fell in love with it and love riding it. Put a le pera daytona sport double seat and sissy bar on it. Runs great and sounds awesome.

 

from the above i am sure you can tell its just a matter of preference... the nightster is a great choice for short distance rides. if you want to be on the bike a good portion of the day, the c has more "comfort mods" available. and actually, i converted my c to a road bike and i can pretty much ride until i run low on fuel- fuel it up and keep going.
so really, imo it depends on how much you want to ride... you also have to dial in your style preferences-