if your talking about a kd wide build, whereas everyone in the kd runs either 15% rax or 15% GS, i would recommend 15% GS. Everyone in a kd will fit enough rax into their prov to have an attack time of under 12 hours so they can hit 2/day. For UD Tact that might not be many, but trying to get ANY kd ghetto or not to attack more than every 12 hours isn't an easy task due to real life issues (only a few kds in the game can get everyone attack army-in-army-out with sub 11 hour attack times). GS as a whole will mean your kd is hard to chain/semi-chain a real benefit.
If your talking about a single player, if they can keep up the attack times, 15% Rax isn't bad, but 15% GS will be more important if they are thinking they'll get chained.