What would be my best option for new wiring in my unfinished garage? It has concrete walls with trusses for the roof structure. There is currently MC cable ran. Would i be better off running THHN in conduit or just run new MC cable?
THHN in EMT is generally less expensive than MC, and much more capable as well (i.e. MC is what it is - cable, 12/2 or 12/3 or whatever, while 1/2" EMT can support up to 4 circuits on 12 or 14 gauge (more wires than that on 14 gauge but you run into derating issues which make sticking to 4 the safer bet for planning.)
If your needs are simple and you don't care to learn the fairly simple process of bending EMT, sticking with MC may be better for you, though personally I find the process of cutting it more annoying than bending conduit for most purposes, but when snaking through joists or the like, the flexibility of cable is a temptation.
Out on the face of concrete walls, or along/under/above trusses I'd choose EMT 10 times out of 10.