Those mag adapters for the 15rd mags work great and are very comfortable to use. They also don't look as ugly as adapters for some other guns.
I'd get a 12rd mag just to have the right mag, and then just get 15rd mags for spares. You have to figure if you need to reload, then you're going to want as many rounds available.
The 6904/06 are one of the most versatile handguns out there. Small and light enough for easy daily carry, big enough to be easy to shoot and actually practice with. Tough enough to take serious use. With the 15rd mags, you can still conceal it (those mag adapters have a rounded back end, and actually help with printing), or use it as a duty gun. They really do stretch into most of the purposes you need a handgun for.
One advantage you have with this over a plastic gun, is you can carry it without a holster if you absolutely had to. Yeah, it 's not a good idea, and I wouldn't choose to do it, but if you had to take it out of your glove box and stuff it in your waistband of coat pocket, you could do it with less risk than a Glock. The plastic guns are great and all, but I'd never carry one, especially without a manual safety, without a holster. With the 6904 you have both a manual safety and a longer, heavier DA pull that minimizes the risk. I don't recommend carrying without a holster, but if need to arm yourself in a pinch, you can safely do it.