I have the Apex kit. It's the best upgrade for the M&P imho. I am a better shot with it.

Reasons to go with the kit:
1. Proven system
2. Easy install if you have a little bit of mechanical aptitude.
3. You already have a vice, something to protect the slide while in the vice (rubber pads, taped wood shims, etc), a punch set including a brass or aluminum punch, small hammer, hex key set..or..
4. Money for all the above + the kit.

Reason to go with the gunsmith:
1. Proven ability. You have faith in. (doesnt appear to be hard anyway).
2. If any of the reasons in the kit turn you off.
3. cost