My daily driver is a clean air machine - either a Tacony-style Dustcare 5600C, a Sanitaire cyclonic bagless, or a Windsor Versamatic. For weekly or bi-weekly deep cleaning, however, it's a direct air Sanitaire.
The reason I use clean air for daily use is because it's much more versatile - it allows me to use a long hose and attachments. That's a must for vacuuming apartment stairways.
A direct air Kirby would let me do that as well, but the process of setting up the hose and attachments on a Kirby is much more cumbersome and it's too heavy to move from floor to floor with any ease.
So direct airs, while superior on open carpet, are reserved for the occasional deep clean in my situation.
I've never used the tandem air, so no input there.