This is a pretty tough decision. Neither are exactly the greatest in-ring workers around, as far as pure entertainment goes. They both know their way around the ring, they both know how to conduct a match and go with the flow. They can both think quickly on their feet, and they both excite a crowd. Triple H has been doing it MUCH longer than Cena, but they've both been the face of the company for about the same total amount of time in their careers.
Now, if this was 2005, or even up to 2007, I would give this one cleanly to Triple H with no argument. But, with his injuries, and with the company's direction towards a more streamlined (boring) formula, the Game's matches have just become completely by-the-book. It's like I could script exactly what would happen. He doesn't deviate from his normal spots, he doesn't change it up every now and again. And even when he snaps and gets pissed off he just gets a sledgehammer and hits a gut shot or something. Triple H is definitely a ring general, but he's also become the captain of the streamlined formula.
Now, John Cena does bring a shit-ton of energy to the ring. But it's pretty much gone within two minutes, and is followed by 20 minutes of him getting his ass kicked, followed by a Hulk-up and an eventual win. So, the crowd is super into the match for about 10% of the whole thing, with the middle just being blah... I guess if you're really into seeing Cena get destroyed, than his matches are great. Look at his Ironman match with Randy Orton. On the outside that looked like a great idea. Those guys were pulling a great feud, and it should have been awesome. But instead it was just a normal Cena beat-down stretched over the course of an hour. However, Cena can pull off the occasional great match depending on who he's working with. But so can Triple H. They're few and far between, but the two can light up an arena from time to time.
All that being said, I'm still going to have to go with John Cena. And I can't believe I'm saying it... I just don't remember the last time a Triple H match even excited me. He's great, and he's a legend, but his matches are just so straight forward and lackluster. I can't wait for him to return, don't get me wrong. But I think that Cena still puts on a decent match even if it's him getting destroyed. It may not be great, but at least it's average (what a horrible way to be described...). Triple H's matches, on the other hand, just usually fall short of the already incredibly low "average" bar Cena has set in recent years.