I am a big fan of the MSI brand. Although I am not permitted to be armed at work, I do have a small MSI spray for the odd "bad dog" wink wink. The flip top canister is much better and safer than the cheap keychain style sprays and is easier to aim and fire. I used to carry the OC and CN they called "triple action" http://www.mace.com/index.php/product/index/id/27
but now mostly use the 10% OC. http://www.mace.com/index.php/product/index/id/32
A couple of nurses in the local hospital ER carry the Kimber Lifeact because they are not permitted to carry weapons of any kind and wanted something that did not look like a weapon. In the pocket of scrubs for example it could be another cell phone, Blackberry, etc.
As for immunity, my F.D. covers the local jail, prison, correctional facility, detox, etc. We had a standby for an inmate that was out of control and he got sprayed with Cap Stun OC. I am particularly sensitive and was feeling it in the hallway as were some of the officers and ambulance crew. One old Lieutenant walked into the cell, picked the inmate up by the scruff of his neck and dragged him to the cot. While the inmate was snorting and choking, the LT had no problems. He told us OC or the old CN and CS they used in the old days never bothered him. He said it was not a matter of geting used to it over the years, but that it had never worked on him ever.