Xstals pay more in the long run, nobs pay straight away but the exchange rate depends on how many lands and buildings you have up. I'v always bought Xstals instead of nobs, I can get over 60bil per day maxing Xstals on Haunting.
Xtals would be better. You get a full unload and a bonus afterward. With nobs you get gold immediately but the exchange rate is horrible and you don't get a bonus.
If u r looking for more hits go for the xtals, if u r looking for gold go for the nobility's Personally I would take both