Kind of depends on the soil you are using now, and when the last time you re-potted. If you are using MG soil, you should wait 2-3 weeks after transplant before applying nutes.

As for the MG nutes, I've heard that they are quite "hot". Start at feeding them once a week with 1/4, and move up to half strength gradually. Any more than half strength once a week might be too much, but some strains use more nutrients than others. Better to add in case of deficiency than to over-fert.

If you are low on cash, the MG nute will do... otherwise the fox farm products seem to be very popular lately.