MicroUSB Car Charger vs. Universal USB car charger?

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Hi All,
Just switched over to this phone from The HTC Touch Pro so all my chargers and stuff are for the Mini USB instead of the MicroUSB, my question is I have a car charger adaptor that has a standard USB dongle that you can then plug any USB cord into...when I plug this in to use as a car charger the phone seems to see it kinda like I am plugging into my PC and shows a little USB symbol in the upper left of the notification area on my phone, also it seems to charge a little but does not seem to keep up, and the phone still slowly discharges when using google maps or navigation, is there something different or better about using an actual micro usb car charger over this universal usb adaptor? Should I spend the $25 to buy a sprint car charger?

Thanks in advance,
Mike