I would go for 1st person voice acting as well. It creates a very immersive atmosphere as long as the acting is good and it suits to the general feeling of the game. If you have a realistic shooter (delta force etc.) about terrorist hunting or wheresoever, I don't think people would like if the character they play speaks like Duke Nukem, especially not after taking out Taliban with an m16, night vision goggles and what else every firearm maniac and wanna be navy seal hides under his pillow. It just wouldn't be convenient, I would use formal military slang instead.