For those who are in between a small duffle and medium duffle, consider the medium travel bag.
Small duffle: 39 linear inches
Medium travel bag: 42 linear inches
Medium duffle: 52 linear inches
I have the medium travel bag and it's been great for overnight trips and weekend trips. It has the advantage of being one of Filson's largest bags that is still carry-on compliant.
But what puts it over the top are the 4 outside pockets and 2 inside pockets (which I can easily fit my 13" MBP with sleeve). It's kind of like a giant 256 but without the outer flap.
The only downside is that price is almost identical to the medium duffle. Plus it doesn't have that "classic duffle" appearance.