I'm hosting a website that the client owns http://abc.com and http://123.com. They want it so if a user types in either of those url's, they will go to abc.com.
Should i be parking the domain http://123.com or should i be redirecting http://123.com to http://abc.com?
What do you mean by parking the domain? Letting GoDaddy (or similar) show a page with "this domain is parked"?
You described that the clients wants traffic from either domain to end up at the abc.com domain. Thats what forwarding is for.
If you're making money from it being parked, leave it parked. If you're not, redirect it.
If you're losing visitors because it's not being redirected, redirect it.
It seems to me that you want to redirect. Domain parking to me is "hey there's nothing here, except maybe some ads".
The client wants all users directed to abc.com, that's redirection in my book.