I guess one important question is have you been satisfied with how each piece of gear has been performing for you. I'm guessing that the mummy bag has been keeping you warm at night. Have you had the GoLite poncho in any bad storms, and if so, how has it worked?
HikerMan2000 made a post today that links to an article by Andrew Skurka and in it he briefly mentions the GoLite tarp. One of his problems with the poncho was it was also his rainwear and the difficulty of setting up the poncho as a tarp in the pouring rain.
Personally, I would go for The HG cuben tarp. It's a great piece of gear and the additional coverage would be welcomed as compared to the lesser coverage of the poncho tarp. Which ever way you go though, you will be gaining a great piece of gear.