Which oceans in North America is warmer, the Atlantic or Pacific and why??

This is almost impossible to answer...

Neither ocean is "in" N. America; it could be argued that N. America is in them...
Neither ocean has a uniform temperature, even if restricted to the coast of N. America (the Pacific off Baja is substantially warmer than off Anchorage for example).

Local variations of temperature are due to:
Latititude;
General climate (cloud cover may prevent warming);
Currents (and to a lesser degree, wind), especially the big thermal conveyors such as the Gulf Stream;
Depth (shallow waters are warmer);
Proximity to land and land mass topography (can restrict movement thus preventing temperature mixing);
etc.

Sorry I can't be of more help...

The ocean along side the east coast - the Atlantic including the Gulf Stream is warmer carrying water from the south while the Pacific along the west coast is generally cooler because there is a cold current that swings in from offshore. The climate of San Francisco and Seattle is cooler on average than the climate of cities like New York and Boston which are further north. The situation is confused, however, by the westerly pattern of winds across the US - the onshore winds on the west coast keeping the cities cooler, but the open ocean keeping them from getting really cold. East coast cities tend to have a much larger range of temps depending on offshore vs. onshore winds.

Neither really.. The warmth of the ocean is determined by it's location in relation to the equator, which is 0 degrees lattitude. In theory as long as the oceans have the same lattitude, they will be very similar temp.