If people really like what they see with the NeXtBox and it has a strong and impressive launch lineup, I think that Sony will have their work cut out for them if they do indeed launch in 2014. Certainly I'm not saying they'll be destined to do poorly, but it could prove hard for them to find their footing in a market with the affordable Nintendo machine (which could possibly have a 20m install base by then) and the U.S. powerhouse from Microsoft, which can easily afford to take a loss if necessary.
If it's within six months then it won't necessarily be a big deal. If someone would rather have a Playstation, they'll probably be able to wait a few months. Christmas isn't usually a big deal for a launch, either, as any console launching at that time will probably be dealing with supply issues.
The key point would be if the two launch dates are close enough that Sony could advertise for the PS4 during the launch of the 720. Say if the 720 releases in November and the PS4 releases in February then I think Sony would be just fine. 2013 or 2014 is less important than the number of months between.
However, a difference closer to a year would be much harder to deal with. Not necessarily a disaster, obviously, as the PS3 recovered nicely from that, but it certainly wouldn't be optimal.