Which toilet to choose? Dometic 320 or SeaLand 510?

310 - plastic, round, 2 parts, inherent flaw - often leaks on the joint.
320 - same thing only elongated.
510 - don't remember, must be same as 310 with the same 2-parts problem, only the upper part is ceramic, Dometic and Sealand is the same brand.

If it's a cabin, you have plenty of space, get all-ceramic one-part Aria Classic: https://janeandjohn.org/pages/potty3.html. It's big (long) but when you build your own shack, this is not an issue.

Or a regular residential toilet, more water but much cheaper.