Are you looking for a 'drop-through' or a 'pump' type head system?
For a pump system, you can't beat the VacuFlush system. They use very little water (1/4 that of a macerator pump system), so your holding tank goes a lot farther until full than any other system I'm aware of. My experience after many years of tracking such use of a VacuFlush for trip planning purposes is 1 gal per day per person, so if you have an 80-gallon tank, it would be good for 80 man days, or 10 people for 8 days, or 8 people for 10 days. My holding tank on my big cruiser is 40 gal, so I plan on 40 man days, but I've actually had 45 man days in the past and have not been full. These systems are installed OEM on most yachts from 30' to +150'.
The con is the system is expensive upfront - $700 for the pump and $400 for the toilet. But like ice on the lake, if you account for travel to/from the marina to pump out, it can pay for itself relatively quickly. If you currently run into full holding tanks in the middle of trips, I'd highly recommend a VacuFlush system - it could extend your trips without worrying about tank issues. If you don't currently have holding tank size concerns, there are other less expensive options others will recommend.