jags,
I have used Blackburn & Bor Yueh copy, Tubus Tara & Ergo, and Thorn low riders in the search for the perfect front rack. I have also used Karrimor, Carradice, Vaude, Altura and Edinburgh Coop panniers.
I have still to try the Tubus Duo, but the credit crunch has curtailed my spending.
The biggest difference has been the ease/difficulty of attaching the panniers to the rack in the position I wanted to put them. Often the rack frame and the pannier clips have clashed requiring surgery and/or compromise.
You really can't go wrong with any of the above, they all do the job.
I have ended up using the Ergo rack plus Carradice Super C front panniers fitted with Arkel clips as the best for me.