I haven't grown either, so I won't answer the poll, but I have smoked both.
I have smoked two different cuts of GDP. One of the cuts of GPD I thought was rather mediocre. The other was amazing. The GPD I liked was dark purple, inside and out, and smelled rather "purple" (like incense & grapes). The flavor was more toward the sweet-grape side, as opposed to the concord grape flavor that I found to be present in the Urkel. It was on par, density wise, with the Urkel as well. The high, although indica-dominant, was extremely clean, and very balanced. It came with a nice body buzz, mixed with a twinge of something cerebral, and enough energy to still run around and take on my day in a regular fashion. It's a strain that is hard to get tired of.
The Purple Urkel I smoked was also very dense, and purple from the inside out. I can't quite remember the fragrance, but the flavor I remember very distinctly as being pure concord grape. The high was more downy, and sedating than the GDP. It was a killer strain when I got it, but after smoking through it for a while, I became very bored with it, and have not pursued picking anymore up.
All in all, the GDP was a better smoke, had a better flavor, and better bag appeal. The only upside to the Urkel over the GDP may be in medicinal effects, but I am not even certain there.
EDIT:
as far as the genetics are concerned, this is what I have always heard:
Purple Urkel is a phenotype of Mendocino Purple
Grandaddy Purple is: (Purple Urkel x
Big Bud)
GDP has been around for nearly 25 years in NorCal, so I hear, so unless Soma's Lavender has been out longer, I don't see that being the genetics.