Hello David - having lived aboard our 58' er for 3 years, I'd say buy a bigger boat!!!!
Cassettes are fine......apart from negotiating sheet ice on a jetty, in the depths of winter,as you go to empty them! :-)
Conversely, in my limited experience of marinas nearby, the availability of pump out facilities seems to be a bit unreliable. The idea of a holding tank ( especially in winter) is very appealing, but as an earlier poster said, holding tanks aren't THAT big!
MY ideal solution would be both a cassette AND a holding tank - having an empty spare cassette is the next best thing!
Sas :-)