On a firearms note, the slippery material of the pockets doesn't work with my pocket holster, so I would assume IWB, tuckable, is probably the best option.
If you want to ever wear a shoulder holster with a suit and not make it blindly obvious, buy a jacket size or two larger than you need. Have the tailor cut it down to fit you, and leave extra room around the shoulders. Usually I just tell the tailor exactly what I want ("I need to carry a firearm in a shoulder holster occassionally, here are the measurements."), and he makes it look a bit more natural. Told one I carried cash in bad neighborhoods occassionally, and he did his measuring thing with me wearing the holster. Did a beautiful job. It's a more expensive tailoring job, but it's worth it if you ever need to carry.
And you can get the pockets replaced with a different material. Heck, I usually do that myself with a sewing machine within a couple minutes. Go to a fabric store, buy two small sheets of whatever material works for you, sew a U to whatever measurements suits your needs, cut it out. Flip inside out. Gently pull the pocket material so it's sticking out of your suit, cut it BELOW the visable level (so it looks like it should) and double stitch your new pocket into place. Very easy, and if you make the correct measurements, your sewing job can look as ugly as sin and everything is still good.
If you go in the IWB, make sure you take it into account when buying the pants. Also, try to go with a slightly longer jacket, so you can lean forward without exposing if you carry in the small of your back. If you need to carry and don't have the cash for an excellent tailor, buy a size up on the pants, and carry IWB on the side. The jacket shouldn't ever ride up enough to expose, and if you keep one button buttoned, it won't open enough to be exposed.