To the OP,
Before you start considering the pros and cons of holding tanks vs waterless (I can't call them composting) toilets, you really need to look at the space you have available.
Waterless toilets take more room than a standard marine head and stand taller. Can you fit one with enough head room?
Holding tanks need quite a bit of space and you need a large access to install it.
We have just gone through this process as our boat had overboard discharge only.
For us a holding tank was cost prohibitive as it would have to be a custom tank and would have to be very small so not terrribly usefull either.
We ended up purchasing a C-head. It is a tight squeeze in our head and we tend to sit higher than is really comfortable (Natures head, Air head and C-head only have slight variations in dimensions). We are still working on a remedy for that. However for us it is a good solution. The C-head requires more frequent emptying of the solids than other brands but the convenience of being able to just lift out a bucket vs taking the whole unit apart works for us. We also like that both containers are inside a secondary container so that if the liquids are not emptied at appropriate intervals the spillage remains contained. We have tested this
and I am very pleased to report that all spillage does indeed remain contained.....
We are currently using a C-head full time in our land based living as well and after more than 6 months have not yet gotten around to hooking up the vent as there is indeed little to no smell and it only seems to smell a bit when it is getting full.
We have previous experience with a land based waterless toilet that did not have the separator and our experience would be similar to parks Canada. It did not work very well, was extremely smelly and completely disgusting to empty. It also took up a huge amount of space....
The C-head is comparatively delightful. I can't believe I am saying that....
We have also had fruit fly infestations. We dealt with the one at home with diatomaceous earth and that so far seems to work. On the boat we hung the bucket over the side for a couple days after emptying and that seems to have taken care of them. I assume the eggs would be drowned or eaten by fish. (I realize this is not an option for liveaboards...)
Best of luck in your decision. Either way you end up getting more intimate with your poo than the normal flush and forget......