OP here. Thanks for the reply.
Re: practice groups, that was not my impression at all. The associates I met with at MB were very much omnivorous. For example, a fourth-year I enjoyed speaking with had cases in telecom and products liability. Unless you're defining "practice group" as the corporate/litigation dichotomy, your statement is at odds with my impression that there is no practice-group specialization as a junior associate.
Re: partnership prospects, NALP relays that Katten, a firm of around 600 attorneys, made 14 partners in 2013. That's probably quite a bit better than MB, admittedly, and elevating better than 2% of attorneys to partner annually is hardly an "infinitesimally" small prospect, especially if you extend the timeframe to 3-5 years. Just saying.
Re: associate life, both firms start at $160k, and I believe both start you with your own office. A 2000-hour billable requirement at Katten with a below-market bonus structure; a 2100-hour requirement at MB with Cravath bonuses once you hit that mark. So that works out to be a 2 billable hour difference per week between the firms, with ample bonuses following at MB.
ATL's
law firm guide paints a stark portrait of both firms. While this may have been prior to the New Republic article, MB's "Insider Rating" was an "A," whereas Katten's was a "C+". The "industry reputation" for MB was an "A-," whereas Katten's was a "B." So I think equating the two in terms of being "second-tier" Chicago might be a bit facile. After all, Kirkland and Sidley are the only Chicago-based firms that would presumably top MB, whereas all three of these, as well as Jenner, McDermott, and maybe Winston, could claim to equal or best Katten's prestige.
Overall, I liked the people more at MB; they were clearly more intelligent with better pedigree, IMO.
It helps to hash these things out, but I'm still unsure. Anyone else have thoughts?