Which M9- did you get the new one with the light rail? If so, a light will be in your goodie box.
On to the holster. Some random thoughts:
On the chest
a. accessible to both hands
b. takes up space on IBA
c. IMHO, more likely to pick up crap from hitting prone (and depending on the holster, the crap is more or less likely to hang up i.e. the SERPA).
d. Weapon is disabled if hit/fragged in the chest plate; possibility of ricochet
On the waist
a. accessible to both hands with practice
b. if mounted to a Brokos belt or similar, can be worn the same way on or off the FOB.
c. can be concealed if required
d. may not be comfortable with IBA
On the thigh
a. accessible to both hands with practice
b. allows your IBA space to be used for ammo and medic stuff
c. may shift a bit when running (what doesn't?)
d. doesn't interfere with prone, and can be reached from prone without raising your silhouette
Looking at it from another viewpoint, I believe it's bad juju to keep changing the location of your pistol. The key to becoming good with anything is consistency in the application of the basics. You need to train from one holster, carried in the same position, over and over so that when you do need to pull that pistol you don't have a brain fart trying to decide where the pistol is. This will also make training for one handed manipulations much easier.
Here's what I do, YMMV. I wear a 6004 Safariland tactical holster on my leg. For a number of reasons, I don't like SERPAs. The 6004 protects the gun, keeps it secure, clears my armor, and because the gun is always in the same position training is simplified. I also don't flag my buddies when training. My armor is slick on the right side so as not to snag the gun. The front of the vest has extra 5.56 mags and pistol mags, but is also dedicated to carrying medical supplies. I tend to work out of my vest (at least at first), so this works well for me. Of course, my AO is straight up big city urban so take all of this in context.
If you go with a leg holster, you have the advantage of one method of carry on or off the FOB. Tip- it shouldn't hang low on your thigh like a cowboy rig. It should be adjusted low enough to clear your IBA so it rides high on your thigh.