I have a 94 pontiac firebird. Is it better and cheaper to get my engine rebuilt or replace the head gasket? I was told my car is over heating and I need a head gasket and 2 freeze plugs.

I would say replace the head gasket.

Or you could just buy another car.

Repairing the engine with headgasket would be relatively inexpensive compared to a total rebuild, barring any additional problems with your current engine.
If it's overheating due to headgasket failure, did the repair shop include any type of warranty for the diagnosis? Did they include a new thermostat and inspect the remainder of the cooling system to insure you don't have pending issues?

It is quite inexpensive parts wise, but labor intensive. BUT, if the motor has a lot of miles and is pretty tired and worn out or is burning oil it may be worth your while to just buy a rebuilt long block. That consists of a rebuilt engine minus intake manifold, throttle body, alternator, starter, A/C, etc. which you take off your current engine and put on the new one. If you are not doing the work yourself, it will initially cost you more for the block and cost about the same for the labor to have all of the parts changed over by an on the side mechanic that it would have cost for the head gasket replacement, in addition to probably $200 to have it installed but you will have basically a brand new engine.

If it needs 2 freeze plugs you need to find out why. If they just rotted out then you are fine but if they are coming out or came out due to the coolant was so diluted or the wrong mixture that it froze and popped them out (that is their intended purpose) then you need to have the block checked out very carefully. I know someone in a similar circumstance whose engine froze and popped a couple of freeze plugs and had antifreeze in his oil so he had the head gaskets changed just to find out that the block was cracked in the waterjacket area between 2 cylinders. Money was already spent replacing the head gaskets and non-recoupable and obviously not by a very reputable mechanic because I spotted the crack right away during the next tear down once the head gasket didn't cure the problem. Also, if you are going through changing out 2 freeze plugs because they are bad, go the expensive of changing them all to prevent problems in the future with the ones that weren't replaced especially if they are rotted out as the others will suffer the same fate eventually.

My .02 cents worth. Short answer: It is cheaper to replace the head gaskets. :-)

BTW: Rebuilding the engine involves a complete tear down which means new head gaskets to put it back together so a rebuild is replacing the head gasket as well as machining, rebuilding, refreshing, replacing the rest of the internal engine components so unless you are doing the work yourself just find a reputable engine rebuilder near you and find out about a rebuilt long block, there is at least one in nearly every major city.