I'm going with Eli Manning, simply because I think he'll be working with a better supporting cast than Philip Rivers. Let's start with the offensive lines; while neither unit is particularly great, the Giants' O-line is a smidge better than the Chargers'. Then, of course, there's Big Blue's superior receiving corps, headed up by last year's rookie extraordinaire, Odell Beckham Jr., whose jaw-dropping entrance onto the NFL scene was huge for Manning. Finally, this will be Manning's second year in coordinator Ben McAdoo's offense, which can only help.
Ultimately, I expect Eli to pick up where he left off in the stretch run of 2014; he posted a 12:3 TD-to-INT ratio in the last six games of the season. Surrounding talent aside, Philip Rivers would be the easy answer. He's been the superior quarterback for nearly every season of their respective careers. Odell Beckham Jr. is a career-shifting advantage for Eli Manning, though, making this a closer call than it would be otherwise.
Rivers' late-season slump last winter was attributable to a painful back injury and a subpar offensive line. With both of those issues fixed in 2015, I believe he still has the edge in this showdown. The draft might change things, but right now, Phil Rivers doesn't have the weapons to compete statistically with Eli. Did you hear the Giants had a good rookie receiver last year? They also have Victor Cruz -- a.k.a. one of the game's better pass catchers -- returning from midseason injury, a talented tight end in Larry Donnell and a now-rangy collection of RBs. After a dicey start in 2014's new offensive philosophy, Manning was able to put together a nice statistical season ... and 2015 will be even brighter.
And if I may transcend the short-term for a moment, I've got bigger news: Whether or not his detractors are ready to accept it, Eli is going to the Hall of Fame. Philip Rivers will have a better year because he's a better quarterback. Eli Manning will probably get pushed into Canton someday despite a career when he's rarely been a top-10 quarterback. Rivers, on the other hand, has consistently been among the best at his position, often among the top five. Rivers' play fell off late last year when injuries piled up, but he's still among the very best in the league at pocket movement and directing the game before the snap.
Everyone's defense of Eli starts with "Rings!", but Rivers would have two rings if he was with the Giants -- and he'd have a lot more than one playoff berth in the last six seasons. I like Eli Manning in this one. Manning made significant improvement in a number of statistical areas last year from his 2013 performance, including completion percentage (from 57.5 in '13 to 63.1), passing yards (from 3,818 to 4,410), TD-to-INT ratio (from 18:27 to 30:14) and QB rating (from 69.4 to 92.1). And seeing how this will be his second year in Ben McAdoo's system, Eli will be even more comfortable with the offense in 2015.
And Manning has better offensive talent around him than Philip Rivers. In addition to getting Victor Cruz back this season, Eli has Shane Vereen joining him in the backfield. Those guys will be major factors on third down. New York still has the 2015 NFL Draft to improve the O-line, something that'd make the offense even stronger.