I understand cassette toilets are permanent installs. I think you mean portable.
This Century looks like a pretty good deal. I'm almost certain it's a Dometic 965.
I've heard good things about
Sanitation Equipment Visa Potty, too.
Thetford is another other option. Also, Fiamma makes a portable, but I found little information when I did my research and few sources.
Here is my take:
Seat:
Thetfords are, I think, too thin at the seat. The Dometic looks a bit wider, while the Visa potty is supposed to have a deeper bowl. Visa Potty wins.
Dump:
Thetford has the best spout (permanently attached), the Visa Potty's spout is seperate (yuck!), and the Dometic does NOT have a spout. This seems a bit risky, but is -- in fact -- very manageable. Thetford wins.
Size:
Visa potty (6 gal) is biggest. However, 5 gallons of waste is heavy, I want to dump as soon as I can after 3 gallons. The smaller (i.e. 2-3 gallon) toilets will probably be too small for your family, unless you dump every other day. Furthermore, the larger ones (5-6 gallon) have better seat height, even for littles.
Conclusion:
I didn't like the seat on the smaller Thetford I had, the larger one retained the seat.. The Visa's spout was a major annoyance, the unit looked cheap, the manufacturer has almost ZERO info on the web, and the flush mechanism looked like it'd waste water. The Dometic could use a deeper bowl and a spout, but I like the construction and seat width. Dometic won.
Make sure you use only BIO-DEGRADABLE chemicals. Never formaldehyde based. You'll pretty much be able to safely dump anywhere (including in a 1 foot trench in the wild).