It you're near Art's, go to Art's. If you're near Jerry's, etc. They're both okay. Otherwise the only true destination deli - where you're going strictly for the food (and in this case strictly for the Pastrami) - is Langer's. I would add Canter's as a destination since it's... Canter's. A true L.A. landmark (of sorts), and not all that bad a deli. Actually for me and many friends, right below Langer's.
One caveat about Art's. I was there over the summer for a solo breakfast, and it seemed outrageously expensive - which delis in general sorta are anyway...
IMHO - not particularly. The chicken soup is okay. The sandwiches are highly overrated. It's good in a pinch if you're in the vicinity, want lunch and want deli lunch - but it wouldn't rank in my top delis in L.A. by a long shot. I prefer Langer's for pastrami and a few other things; Brent's for all around. I even like Canter's better than Art's.