Carradice definitely
I have the Carradice SQR Slim, it’s much better than a bikepacking seatpack for commuting use (I also have a Revelate Viscacha)
– Quicker to get on and off
– more robust (and it maintains its shape when half empty, it’s not floppy)
– easier shape for packing things it – laptop, shoes, shirts, food etc
– you can clip a rear light onto it more easily
– easier to carry when off the bike
The bikepacking seatpacks probably look better, but they’re way more faff
I use neither for commuting, I use a Topeak rackpack combined with a Topeak rack. But the Carradice got used when I got my brompton and before I bought a Brompton bag, I leave the SQR mount on the Brompton for just in case use