First off, poor Seoul. Presumably, the Commies have the initiative and start hostilities. I can't see them getting close to any sort of readiness without us matching, so everyone was coiled, and they sprung first. China would attack any Allied fleets off their coast with cruise missiles, DPRK would move south, and the PLAN would surge boats out a little past the coast. China's ground forces can't really do much, and they don't have the capacity to launch any major amphibious operation.
We would decimate both the Korean and Chinese air forces, get air parity, and move USPACFLT west. Korea would be a meat grinder. We'd reinforce round the clock. It'd actually be similar to a USSR vs NATO scenario. the Allies try to reinforce the ROK, and keep the SLOC open. The Commies try to deny us reinforcements, and win the only real ground war. Everything else would be naval/air oriented.
I'd say we'd win. Same reason as in a Cold War escalation scenario. Technology and firepower over pure numbers. Our advantage is even more pronounced now. At the end of 2013, it wouldn't be a contest.
10-15 years down the line, it'd be closer, or it might go the other way. I'm not informed enough to say more than that.