I have a Mission Workshop Rummy, my friend has a Seagull. I think the the Mission Workshop is the better bag overall. There are more compartments, and the rolltop feature offers more versatility in how one is able to carry things.
However, when I first got it, it took some getting used to, given there is no velcro to hold the main flap down- it has to be buckled over, or rolled up. Now I don't even think about it, but when I first got it, I felt it was a hassle, as I was used to the flap-down velcro on most messenger bags. (Again, now it doesn't bother me at all)
If you are looking for a great laptop bag, bear in mind Mission Workshop does not have a laptop compartment, nor padding on the back of the bag.
Seagull does offer these features (padding on the back of the bag is nice), and they have the big flap with durable velcro, but they have fewer compartments to hold things. There is a small pocket inside the main bag, but otherwise there are only two bellows pockets on the bag (though they are nice and deep).
I still think Mission Workshop is the superior all around bag, but a laptop isn't the main thing I'm carrying.
Have you checked out Bailey Works? They make excellent bags, The Super-Pro has way more compartments than Seagull, and offers an integrated laptop sleeve, which is so nicely padded, it acts as padding for the back as well. I also have one of these, and when I'm carrying my laptop, I tend to use the Bailey Works.
Another option, of course, is Timbuk2. They make bags specifically designed for laptops, with easy access and everything. Timbuk2 bags don't ride as nicely on the back as Mission Workshop, Bailey Works, or Seagull. Also, most are made in China or Taiwan now, if that matters to you. Mission Workshop (CA), Bailey Works (NH), and Seagull (OH), are all made in the US.