I use the Versa-Max 2 holster which allows you to use either Kydex clips or snap-on belt loops as you prefer. I'm a big fan of the snap on belt loops myself. The clips for the Versa-Max are very good but for a lot of holsters (my Fobus, Galco Deep Cover, Shooting Systems ballistic nylon junk) the clip leaves a lot to be desired.
The only advantages of the clip are that you don't HAVE to wear a belt and some clips (very few actually) allow you to tuck your shirt in over the top of the gun for better concealment (Galco Deep Cover, Versa-Max 2 are two examples). Some clips still require belts to work effectively (anything with a J-hook clip) though.
The disadvantages are that your pants are generally not strong enough to hold up a full-size handgun with a full clip and a belt will be a desired item anyway. You can wear a belt over the clips to help support the gun. The belt will conceal the clips and make it less obvious you have a holster if part of the belt is exposed.
The J-hook that Galco uses and the majority of clip-on holsters also are very poor at keeping the gun in the same position and location (although this isn't true of the Versa-Max and other quality holsters like Alessi).
I'd recommend snap-on belt loops and an IWB holster that has at least two loops widely spaced for the best combination. The only disadvantage of the loops are that they work best when sized exactly to the belt (meaning you buy a quality gun belt from the same people you buy the holster from) and if part of the belt is exposed, people might notice the loops. Otherwise, they generally are better at stabilizing the gun and are easier and more comfortable to use in my opinion.
