I have both the Salsa Air and Alum (Topaz) as cabin bags. I prefer the Topaz, but have to be very careful not to get over the weight limit for carry ons... ironically it's more comfortable to push around due to its weight. My Salsa Air got a crack after 5 years which cannot be repaired. Can't happen with the alum. The Salsa Air on the other hand is very light. So you have to consider what's more important to you. Also if you prefer to have no extra bags on board the one with the zipped compartments outside might be practical. Never considered it as I always take an extra bag.
After consulting with my better half, we are leaning towards the Limbo which is the poly with the reinforced alum corners. The only thing that I'm worried about is the size for US domestic carriers - the max on United and Delta appears to be: 14 x 22 x 9.
The Limbo (actually most of their cabin bags) appear to be 15.7 x 21.7 x 7.9 or 15.7 x 21.7 x 9
Have you had any experience of fitting your carryons into the airline sizers and have it fit?