On A Garmin Gps (c340)what Is The Difference B/w Using A Traffic Antenna: Gtm 10 Versus The Gtm 12, Just $?

Also, I just bought the Streetpilot C340, It is preloaded, but I cannot download updated maps because it says I don’t have Mapsource. I cannot download that because my garmin was preloaded… how do I add new maps if I decide to buy them?
Oh, and yes, the antenna question please.
Thanks

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