For pumpout, you must have a holding tank. That could be part of a portable system or in the case of the manual flush option, a separate system.
I suppose there advantages and disadvantages to all options. With respect to resale value, for larger hulls (greater than 20 feet or so), having only a portable MSD will hurt the price. Have a manual or electric MSD is expected on larger boats. Electric discharge from a holding tank doesn't mean that the toilet has to be electric. One advantage to an electric toilet is that the output pump usually has a macerator pump which grinds up solids and that makes the plumbing less likely to clog.
Vacuum MSDs are a different story and have several advantages including the ability to work with a small amount of fresh water. The downside is that they are more complex and have several external components. that have to be mounted under the deck or in storage space.
In Mike's case, he can opt for electric or manual but he has to have a holding tank and if you are doing that, you might as well get one with the overboard discharge option. It can aid with holding tank flushing on the land when doing seasonal maintenance.
As was mentioned, if you think you will be using the head a lot and for both liquids and solid wastes, An electric head with macerator and a holding tank is probably your best option.
f there is an option for fresh water fill, you may want to go that route primarily to reduce odors but if properly maintained, odors shouldn't be a major issue just because you flush with sea water. If that were the case, a lot of livewells would stink ( well maybe a lot do
)