They're pretty similar in r-value (cellulose actually being a bit better), so go with the cheaper option. (Have you taken into account the cost of equipment rental to blow in the cellulose insulation?)
Also, keep in mind, this is a garage, not a house. You shouldn't need to keep your garage at 72 F, (maybe 60), so I'd save the money, gain the space and go with 2x4 walls......especially with radiant floor heat, which heats objects, not the air, which will make you feel warmer than the air temp anyway.
IMO
Here's some more info....
Cellulose vs. Fiberglass 
