i have both shafts.....
the i2 has a long comfortable taper, and hits pretty solid. given the taper, the shaft isn't whippy due to the graphite core. the shaft is on the lighter side, which might throw off your cue weight and balance. without an insert they normally weigh around 3.3-3.4oz. this shaft does deflect quite a lot though, more than most standard maple shafts. the design carries quite a bit of weight in the front end causing a fair amount of deflection.
the g-core has a stiffer taper than i2 and deflects a little less, about the same as your standard maple shaft. the weight on average is heavier than the i2, but they vary quite a lot depending on the wood. it does have a red fiber ring between the ferrule and shaft, which i suppose could be distracting to some.
depending on the tip you use and the butt, the graphite in these shafts can sometimes make a ping sound. overall, the i2 has a crisper hit, the g-core slightly quieter. probably due to the ferrule material...i2 has a linen based hard material, g-core is juma and also has 2 pads.
if you are used to a standard deflection shaft, both are decent choices, though I find the deflection on the i2 kind of high. i like the crisp hit of the i2 though better than the g-core.
everyone has different tastes though, so i'm sure you'll find other opinions too