A lot of ceiling speakers have Pivoting (Swivel) Tweeter (Aluminum). Woofer is also Aluminum.
1" Tweeter, 6.5" Woofer, 25 YEARS Warranty
1" Tweeter, 8" Woofer, 25 YEAR Warranty:
To me it's more important to match all my speakers. If I had all ML bed speakers, I would get all ML ceiling speakers. If I had RBH bed speakers, I would get all RBH ceiling speakers.
I also like bigger drivers for more dynamics and power handling, but I think at least 6.5" woofers should do. Driver material is also important to me - Aluminum Tweeter and Aluminum Woofer.
Warranty: Monitor Audio Lifetime, RBH 25YR, Martin Logan 5YR, Definitive Technology 5YR.