Oh man, tough choice. One I couldnt make myself, so I have both.
I built the AR from parts and its a fine firearm. Out here on the farm its good for about anything from target shooting to coyotes. I like the versatility of it and abundance of accessories. I buy ammo in bulk and last bunch I bought came to about 32 cents a round for Lake city 5.56. Mounting a good scope is easy with a million choices.
The Garand is a fine shooter, unapologetic in its forged steel and walnut manufacture. To me it represents a different era and is steeped in history, perhaps even molding history as it was the main battle rifle for US forces from before WWII, thru Korea and into Vietnam. Once you really get into one you really appreciate the engineering genius behind its design.
The Garand comes with military peep sights, which can take a little gettinmg used to. Scope mounting is quite a bit more challenging due to the top-loading, top clip ejecting construction, but can be done.
Ammo requires a bit more research than picking up some .223 for the AR. I recently purchased 200 rounds of Greek mil surplus M2 ball 30.06 for just under $100, which seemed pretty reasonable.