They seem to have the same price but somehow incomparable...
The AMD seems to be way better than Intel. So please help me out.
it all depends on your requirements and preference. either cpu can be suitable for your needs. since you haven't specified what you want to do with the cpu or just plain curious, it's hard to give you a definite answer.
comparison depends on the criteria.
for example, the fx 8120 will beat the 3450 at highly threaded tasks like encryption, cpu based video conversion tasks etc. it will perform even higher with overclock.
on the other hand, the 3450 will beat the 8120 at gaming with a powerful discreet gaming gfx card even if the 8120 is overclocked (app dependent). it'll also be more power efficient than 8120 requiring lesss cooling than the 8120. the 3450 will beat it at single threaded tasks like itunes encoding.
the fx 8120 is fully unlocked, so it can be overclocked easily (disregarding performance). that cannot be said for partially locked 3450 - which can be overclocked 4 bins above stock clockrate, with a motherboard chipset (p67, z67, z75, z77) that allows cpu overclocking.
if you include the igpu in the comparison, the 3450 beats 8120 as the 8120 has no integrated gpu.