The memorex true 8x is a pioneer DVR-107D. And according to my information it requires 8x media to write at 8x - just like the pioneer DVR-107D.
I do not have the sony myself. But I know that with firmware 2.0a it's one of the faster DVD+R writers due to P-CAV writing tech at 8x speed. 8x DVD-R writing is still done with Z-CLV writing tech tough. (information comes from Ian @ www.cdrlabs.com)
The drive is made by Optorite and is a Optorite DD0401 with an improved firmware and a bit differnt front bezel if I'm right.
I don't know which one of these to recommend over the other.