If "both" includes the MEC rack, that one doesn't have a light mount at all. I have one and it's all right but I don't like it.

I've never used a Topeak rack. They have good reputation. I have a Tubus Logo and it's rock solid. It actually makes the rear triangle of my bike more solid. The lower rails make putting the panniers on and off more easily when there's a trunk bag on the platform. I also like my rear light to be bolted on. The Cosmo didn't exist when I bought my Logo or I would've chosen that one for the wider platform. The Vega would be enough for credit card touring and commuting, and probably even "loaded" touring. The platform is a bit narrow in front and the lack of support in the back could be a problem with panniers without a rigid plate.