I went with the Boulder Boots. I figured since the Lems are borderline minimalist, with a 12mm stack height (9mm sole + 3mm insole), I mis' well get the shoe I'll use in winter when I might be willing to sacrifice groundfeel for greater insulation. If the Mariner's sole were more in the 3-6mm range, I would've chosen them, as I'd probably get more use overall out of that sort of shoe.
Still, I wonder how often I'll have the motivation to deal with all those boot laces. That's quite a commitment. And I've been getting by just fine with the Neo Trails as my winter boot for things like sledding with the kids, so it's kind of capricious to get a real boot. But it's free, so what the heck.