Obviously, you would get a lot more volume with the mini stack with two 10in speakers so if volume is a prime consideration then the Valbee loses out, no question. For your needs using mainly the clean channel with pedals in front of the amp, I think the Valbee edges the Marshall only slightly (bias aside - I own a Valbee as you already know), but again when you turn it up, the Valbee loses out because of the smaller speaker. The soundstage will not be as big as the mini stack.
Forget about the high gain sounds on the Valbee. Even though it was labelled as such, you're simply better off using pedals. BUT, the clean sound is a very nice platform for pedal-induced tones. The effects loop is an added bonus as well.
Don't think it's an apples to apples comparison, though they're both still in the realm of being home practice amps. I'd say the mini stack is more of a living room amp -- it will shine more if you have a bigger space and can crank it up a bit more to make full use of the two speakers. The Valbee excels as a bedroom, under-the-computer-desk type of amp (which is how I'm using it now). Don't get me wrong, even with the 6.5in speakers in the Valbee, it is still very loud for bedroom use. It can get decent low-end via the sealed and ported combo enclosure, but of course, don't expect the low end response that you would get from a 12in speaker. A tip with the Valbee is, if you want more low-end response, position the back of the amp nearer to the wall, or against a corner for even greater effect.
Oh yeah, if looks are a consideration as well, you can forget about the Valbee.
