Those are some good posts, Kelo6
I really like airsoft for their realism. Yes, the KSC/KWA Glocks may or may not be an exact fit into some of the more tightly molded holsters, but honestly, leather and nylon can be "broken in" to fit, and Kydex can be easily adjusted with just a little heat applied. No big deal. To me, the realistic operation of the replicas outweighs these relatively minor gear concerns. If you're really worried, just by gear that's more generic (which doesn't have to be bad/inferior gear, BTW) - many airsoft gamers use Safariland or Bianchi holsters and gear.
The negatives of airsoft?
The weight distribution is off. Even with a "metal slide" and "metal" outer/cosmetic-barrel retrofit (which is something that, today, many of these replicas already come with from the retailer - back in the late 90s and early 00s, when I was active in the hobby, these were significant upgrades!), you're still going to notice the difference. The Glocks will still "point" like they're Glocks, but they won't handle quite the same way as your real-steel.
The action, while realistic, offers blow-back "recoil" that's no more significant than a .22-caliber trainer. Although your desire is for these replicas to serve as FoF traiing tools, this is also not to say that you cannot work on the basics of marksmanship with these items: look at who won the 2004 Steel Challenge (and who he competed against). You just have to train with the proper framework in-mind, with the correct goal.
You'll need to adjust your engagement distances. A really good airsoft replica can chuck a 6mm plastic BB with better than minute-of-man accuracy at well upwards of 10 to 15 yards, but you'll need to remember that there will be a difference when compared to the real thing, and even Simunition. Static and at shorter ranges, this may not matter, but as you start moving faster and as you start to stretch the engagement distance, keep this in mind. Otherwise, you can expect those who "game" to the training to start pulling The Matrix
You may not feel the impact of the 6mm plastic BBs. Yes, at closer ranges, these 6mm plastic BBs can do some damage: chipped teeth, lacerated lips/nostrils/ears, bruised/broken fingers - these are all very real possibilities, and you should make effort to be sure that your FoF participants are safe from these hazards (including the obvious - eye protection) - but as you stretch range and/or under the influence of an adrenaline dump, it's very possible that participants will not feel the BB impact, especially if you have them wear hoodies or the like, to minimize the chances of bruising/skin cuts. Good airsoft players are attentive to the sound that BB strikes make on their gear (of which they're often decked-out in the latest and greatest), but if you're at closer distances and can keep your clothing relatively light, you should be able to feel impacts.
For better or worse, this is where I prefer the RAP4 trainers. Their larger caliber definitely puts out more of a hurting, and while I am certainly not one of those who think that good training should hurt (I'm too old for that kind of macho-think! :lol
, I do believe that it adds an element of stress to the training, and that it's a good one. Using the rubber balls instead of the paint, it's no messier to clean up than airsoft.
The only big downfall of the RAP4 trainers - as compared to their airsoft counterparts - are their pistol trainer magazines, which are "stick" shaped and really hard to index. Certainly, the airsoft is not without fault here, either: their heavy magazines are prone to drop-damage. While neither are necessarily deal-breakers, as most FoF scenarios expire without a reload, these are still issues worth considering. With airsoft, at least the magazines fit into "standard" support gear and are more or less approximately weighed to a full-up real mag - indexing for and practicing tactical reloads and reloads with retention offer more realism with these magazines than the RAP4 pistol trainers. [ Here, I have to make known that I have not played around with the RAP4 rifles enough to know how well their magazines work.
"Realism" only goes so far, though - neither airsoft nor the RAP4 trainers will give you realistic simulations/practice of semi-automatic weapons malfunctions. Even a press-check isn't something that can be done, here.
Hope this helps!