Since the magwell is in the receiver, the receiver type is what determines magazine type, not the kit. The inch mags have a large lug attached to the front while the metric mags have a small stamped out projection. The large lug won't fit in the smaller metric recess, but the metrics will fit in the larger inch. It will rattle around but usually does not cause feed problems. OTOH, the vast majority of mags out there are metric and they cost less.
As far as the kits go, I prefer the Inch style pistol grip and the selector is a lot easier to use. The hammer assembly is also captured, so parts can't go flying too far.
BTW, inch and metric are just a shorthand way of differentiating between the types. Both bbls have the same threads and will interchange on the receivers. Lowers interchange as well, although internals vary a bit as to pin size and such.