Great question. Tough one as well.
OJ McDuffie was a more complete WR in the route tree aspect (or at least he was utilized as so), had better body control at the catch point, and had a penchant for big time catches, especially in the endzone. That said, he also a bad case of drops when it come to the average catch. Often not honing in when he was faced with the usual as opposed to his ability to focus and make a spectacular catches along the sidelines, etc.
Jarvis Landry. A victim of his own success in our quick passing game? Relegated to the short field b/c of other intermediate-deep receiving weapons? Maybe. Or possibly he just is limited to the short-intermediate passing game. Regardless, great competitor, willing teammate, and very good all-round WR who can also be counted in the run game. Also very reliable and catches anything thrown his way. Biggest knock is he's not really a threat to score in the redzone as most of his TD come via YAC outside that area.
Overall, I loved OJ's highlight reel, but Landry to me is the better all-around reliable receiver. I'd have to go with Jarvis Landry here.