Yeah, the whole "geek the mage first" is an adage and is in no way like hard coded AI. The GM is in no way obligated to not have the opponents attack you first just because you don't look like a mage, or rather, not give you less of a chance than the rest of your team to not be attacked because you're not magic looking. Heh, I mean, if no one in your group is obviously a mage, its not like the opposition will delay their action waiting to see if the mage reveals themselves, they'll prolly just attack whoever is in front of them, and at that point they might decide to focus fire on the armored street sam and take her out before she can start killing them.
I mean, if you want to run around in full mil-spec armor for the protection, sure. (though running around like that is sure to freak the opposition out, and if they're corporate/governmental, they might call in heavily armed response teams with anti-armor capability) If you want to carry around an AR or shotgun to have more tools in your role as a "generalist", that's fine too. But if its just to not look like a mage, that's a little much.
That being said, you could always carry the Optimum II or an HK XM-30 (configured with the underbarrel shotgun). Then you'd have both on in one handy weapon. But yeah, they each have their role, and honestly which one is better depends on the mission. An AR might be better in general because it has better range- yeah, its not as powerful close up as a shotgun, but its no slouch either in the damage department. So, you have a way to engage enemies from farther away if you needed to. Plus like some people said, you can use it for suppressive fire. Of course, my street sam alternates between the SPAS-24 (w/ gels and flechettes) and the EBR (with gels and APDS) depending on the likely opposition and the environment, so she can't do suppressive fire, but at least they use the same skill.
Again, hopefully you are asking this as "which one will be a more useful tool" and not, "which one is the one a street sam is more likely to carry" as part of your disguise, like if you picked wrong the opposition would be all, "What the, they're dressed like a street sam, but they're carrying the wrong type of weapon for this situation, something's fishy here, they must be the mage, geek them!"