I'll second (or third) the recommendation for the Buzz Kidder Rob Roy - I like it especially because it's roomy, has a easy open/close magnetic clasp, and I got mine in brown (which to me is a lot more useful for casual wear than black, since almost all my leathers are cordovan or brown - my only black leathers are among my most formal "stuff").

That's of course if you don't like this sporran with the US Army tartan.

BUT having seen (reviews and pictures of) the Thorfinn sporrans, I'd have to say I'd be tempted by one of them, too... as long as it was brown or cordovan (again, my bias towards color).