Gosh, I love them both! I have a 35cm potiron canvas GPT with leather trim that serves as my work horse. That bag gets shlepped to work everyday and serves as my carry on for air travel once a week or so.
But, I also have a 30cm swift leather GPT that is so beautiful and practical!! However, because the interior is lined in silk, I wouldn't use this one as cavalierly as I use the canvas one.
Being the rabid GPT fan that I am, I also have a rocabar (wool) 35cm GPT and a 20cm lamb leather bolduc GPT. I love them all and wouldn't part with them!
Didn't help much, did I?