Two locks= four insta-dots and two fears. AND two soulstones for four wipe recoveries an hour.
Two mages=two crowd controls and a bit more quick dps.

I think it really has to do with your play style. I enjoyed having FOUR locks because of the 8 insta dot factor.

If I had had TWO mages waiting in the wings at the level I am at now I might have gone for the two mages and one lock. As it stands I am enjoying having the TWO soulstones.