The idea is to get the mic as close to the individual as possible not only to eliminate the majority of the ambiance but to also reduce the noise floor(most important). You will find that a good boom operator with a good mic and setup will have a better sound and lower noise than lav mic'ing everyone and mixing them down. I say go for the k6/ME66 with a good carbon fiber boom pole and a blimp(no windshield for indoors). Every time i've done a interview i've always mic'ed both ways but usually end up choosing the boom setup audio in post. Which leads me to point out that you might do what i normally do and lav mic everyone and mix it down to one channel and throw it on the left channel and then send the boom mic to the right channel. In post you can choose and if needed you have a alternate backup.
Another option i've used a few times is to use my VP88 for micing two people at once. The VP88 is a stereo mic but uses MS encoding which can be converged to mono in post and has no phasing issues.