They mean the same. Hold up is a bit 'hipper'.

to hold on = agarrarse bien; asirse; esperar to hold up = atrasar; suispender; sostener; levantar; atracar; presentar

"Hold on (to)" can mean to take possession of something for some time. Please hold on to this for me until i get back from my trip. Note that "hold onto" can also be used instead in this case. "Hold on" can mean hold tightly. We held on as the train began to move. "Hold on" can also mean to wait. In telephone conversation, this is actually asking the listener to hold onto the line and not put down the phone yet. The use of this words has shifted from just waiting on the phone to other uses in daily life when asking people to wait a moment. Hold on Jason, I'll be right there. "Hold up" on the other hand means to be delayed. I was held up by heavy traffic. "Hold up" is hence sometimes used in the more colloquial sense to mean wait up. "Hold up" can also be used in the sense of a bank heist or armed robbery. Everybody get down, this is a hold up. Hope this helps.

In my opinion, a word to "hold on" something is used necessary personally for you, and to "hold up" you mean to someone is necessary. At me such associations :)