I have the stainless CNMG conversion kit. It works pretty well in my AR's and it's intended as a training aid and it lets me shoot my AR's at an indoor range when my local outdoor range is closed. I'm 100% satisfied with the CNMG conversion kit.
It is however not as good as a dedicated .22lr such as the Ruger 10/22. I've got two of the 10/22's and they are great guns and the accuracy is MUCH better than an AR with the conversion kit. The rifling in the 10/22's is intended for .22lr while the AR is not.
I've heard that you should shoot some .223/5.56 through your AR after using the conversion kit in order to clean out the gas tube. I do it. I don't know whether or not it's truly necessary but it can't hurt. I have seen a pic of a badly leaded up gas tube on
www.ar15.com or
www.m4carbine.net a while back but it may be one of those one in a million things. I've also read that you shouldn't shoot non copper jacketed bullets in the AR's when using the conversion kits.
I like the conversion kit but if I seriously wanted to shoot .22lr for accuracy instead of just playing with my M&P15 at the indoor range I would bring my 10/22's. I like the idea of knowing that even if we have another ammo drought or prices go crazy I still have ammo for my AR's and I can still train with them. The groups are really lousy though once you get past about 50 yds and a dedicated .22lr rifle is a better choice if you are serious about target shooting that day.
BTW, CNMG has (or will soon have) a conversion kit that has a bolt hold open device. The model that I have does not have this but it has never been that big of a deal. The training would be more realistic however if the functioning was closer to that of an AR using 5.56mm instead of .22lr.
I got my conversion kit from CDNN. They had the best price, cheapest shipping and they always get your gear to you quickly. I've used them several times and have never been disappointed.
I will say that these conversion kits seem to be inconsistent with some rifles. I've seen YouTube vids in which shooters cannot get their M4(gerys) to run with them. I saw a Colt shooter at the range have failure to fires constantly with his kit. We tried it in my Bushmaster and M&P15 with similiar bad luck. Mine has been good to go however.
Anyway...this is my take on the topic. I've got both of the weapons you've discussed and like both setups. I guess it depends on what kind of shooting you want to do and if you're willing to risk getting a bad kit (or having a rifle that won't work with a good kit). I'm far from being an expert and I hope a few other shooters will weigh in on the topic.