Many USB dongles do not have Mac driver support. Either they don't exist at all or you are own your own finding something "fourth-party" that works.
People buying Mac-capable dongles from OWC have reported good results. I like this one:
http://eshop.macsales.com/item/Newer%20Technology/MXP2802NU2C/
because the "cradle" allows flexibility in placement of the receiver for best reception.
Whether to get a dongle that does "n" speeds depends on your connections. I have a MacBook Pro with "n" and a PowerBook G4 with "g." Our cable ISP delivers 5.5mbps in our modest northwestern town. Both computers report the same download speed at home. Only when I packed both to Portland OR last summer to visit our daughter, who has 24mbps Comcast service, did the difference between "g" and "n" showed up.
The PPB17 showed it was hitting about 17.5mbps and the MacBook Pro plus my daughter's Mac Mini reported a little more than 18.5. All three computers on a cat5 wire reported 24.
"n" is supposed to give better range from the router than "g" but, other than that, you won't see much difference between the two unles you have an broadband connection fater tehn about 15mbps/