Have heard great reviews of Lavac, but we had them on a charter cat a few years ago and there were problems with both of them. One shot toilet water upward all over the inside of the closed seat every time it was flushed, the other wouldn't release the vacuum for at least 5-10 minutes after someone had flushed it (which meant nobody else could open the seat to use it after them). It's probably the simplest design.

I had a Jabsco on a prior boat and it was the cheapest-made piece of junk ever.

I've been using the Raritan PHII (compact) model for the last 10 years and have had no problems. However, they do need to have the joker valve changed and be greased internally every year or two. It's crappy maintenance but it's got to be done. (I use Teflon winch grease and smear it all over the inside of the pump while wearing a disposable rubber glove. Works like new after that.) BTW don't get grease on the rubber gaskets, or they get forced outside their proper position, from the inner pump pressure.

Edit: I originally wrote Vacuflush but I meant Lavac