You say: "The boutique doesn't do insurance defense, only really interesting work from institutional clients on the defense side from what I can tell."
Lots of questions here. How can a firm be doing institutional defense work without doing insurance defense work? Virtually all institutional defense work IS insurance defense. Even corporations that use large self-insured retentions, like FedEx, operate exactly like insurance defense set ups. They even hire from the same pool of insurance claim adjusters to handle claims.
You also say this is "interesting" defense work. Not sure what that means, but after several years of being a (now former) insurance defense associate maybe I'm jaded. Products liability? Car accidents?
You say this about the other firm: "The big p-side firm is one of the most pre-dominant in MDL mass torts in the country, and they do securities litiigation, qui tam, etc."
Ten years from now when you get nudged out of BIGLAW because you are among the 95% of senior associates without a massive book of business, what are you going to do? Open up a solo shop doing securities and qui tam litigation? You could but that won't last long. On the other hand, you could spend your ten years becoming a skilled, gritty civil litigator whose skills can translate to any number of areas of law, in which case maybe the USAO or a state attorney general would want to bring you aboard. And for that, you might be better off going for whichever of these 2 firms offer you earlier and more hands-on experience, and avoiding the one that will stick you in a back room for 5 years writing memos.
Insurance defense sucks in its own ways so I get that. But as a young insurance defense associate, I knocked out about 200 depositions in my first 3 years of practice, feel perfectly at home in our state's appellate courts, and the list goes on. A buddy of mine who went to Yale the same time I went to my T50 law school, in the meantime, didn't see the inside of a courtroom until his 4th year of practice. (And I still don't really count that. It was a mundane status conference where all he did was announce his name and sit down. But he was excited about it!)
Years later, when it came time for me to give up the insurance defense world, I applied for a government dream job. I remember one of the panel interviewers asking me if I was the kind of lawyer who he could just hand a case to and then not have to worry about micro-managing or supervising because I automatically knew what the hell I was doing. It took me all I had to not answer with, "Dude, seriously?" I got the job.