In VERY general terms, more cavities = more isolation. Larger cavities generally have a better efficiency - ie: less loss through the duplexer for equivalent isolation achieved.
As Berkinet mentioned, the higher quality duplexers involve a combination of bandpass and reject technology. Lower cost "notch" style duplexers are usually limited to 65 to 75 dB of isolation between transmit & receive. Notch duplexers suffer in high power use, and they won't filter out other nearby transmitters very well (They are designed to only "notch out" the transmit frequency of the machine they're used on).
Good duplexers have high isolation, low loss, low noise, and can handle high power. They'll also use quality components, and will be built to handle large swings in temperature without de-tuning.