if you're just coming out, i'd recommend trying both. they've both got lessons to offer.

I find these sort of decisions are hard to generalize, yet I'll make an attempt (based on my experiences):

a boutique office will probably foster better/closer relationships among colleagues. The chances of making meaningful friendships, finding a 'mentor', and ultimately learning will thus be encouraged. Your employer and/or colleagues might also be better connected to academia than their corporate counterparts through part-time teaching gigs.

Then again, a corporate office generally casts a wider net. There are more employers, so 'making connections' in the industry might be easier. Additionally, a larger office has probably dealt with more interns/recent grads/young designers (assuming that's what you are?) and may know better what to expect of you. Also, they will most likely have greater resources in terms of available training, tutorials, and opportunities for prof. development. You'll probably be exposed to a greater number and variety of projects (although your role within them may be more specialized / narrow)

And of course, a big thing is just differences in basic office culture. My office could be described as 'very laid back'. We have no prescribed office hours (show up when you will, take your lunch when you will, leave when you will), the dress is casual, and the atmosphere is a studio environment with a very flat hierarchy. Generally, I find that a good thing. Of course, that 'looseness' has definite drawbacks as well. With no office manager, secretary, or HR person, things can get quite messy...Especially when 10 guys want to be designers, but nobody's interested in taking care of the paperwork or finances. Additionally, there's a tendency to work insane hours nearing project deadlines (which is fairly universal in architecture granted, but *might* actually be recorded and thus compensated for at a more corporate firm).

I have done both as well... I enjoyed the several years I spent at the boutique studio but it was a lot of hours and a lot of unbuilt works. The stuff that was built was a tremendous learning experience, we actually suggested means and methods for much of the work because most was custom. I was paid well but it wore me out... I got tired of reworking everything for being over budget or not constructible. The clients are great, they tend to be more forward thinking and diplomatic in their approach and oversight.

The corporate gigs were fine too. I still worked a lot of hours but for different reason. Most corporate firms stick to standard detailing and cataloged products... not too exciting.

Right now I am unemployed and I find myself sending resumes to corporate firms. I have developed other interest in architecture and other disciplines and I feel a corporate firm will allow me to have my own life and time to explore those interest. I also want to build my own practice at some point and its a lot easier to moonlight while doing corporate.

Just my experience...