Factory Nav vs Aftermarket Nav

my 2007 Tahoe LTZ came with rear DVD but no Nav.  I want to add Nav to my Truck.....should I go with after market or with factory......is adding a factory system difficult?
 
I found this system on ebay......would it be as simple as unplugging my current unit and plugging this one in?  will i retain all my functions?  ie steering wheel controls, XM radio, rear DVD?
 
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=250182832634&ssPageName=ADME:X:RTQ:MOTORS:1123
 
I could also purchase a new one for about 2x the price:
 
http://www.gmpartsdirect.com/results.cfm?singlepart=1&AddPartNumber=19153969&partnumber=19153969
 
If difficult to instal on my own what would GM dealership charge to swap out?