Well I've flown the 503 and Jabiru engine fits, and enjoyed both. If there are any other engines available, I'm afraid I've not flown them so can't offer any opinion.
But, frankly if I had an even choice of a 503 or Jabiru version, that's no choice. The Jabiru (once it's working properly) may give you almost 30kg less payload, but it's quiter, smoother, less thirsty, about doubles the climb performance and shaves a fair bit off the take-off distance.
And you've still got enough for 2 large adults and full fuel anyway - so that would be my choice every time.
The one thing is I really don't like about the type is the electronic pitch trim, if you are building one I'd go for the mechanical cable trimmer, which I find much easier to fine-trim with, although it can take a bit of mucking about to setup right at the start. (One other building hint, put the compass bracket exactly where they recommend, it puts it exactly on the horizon line for a perfect 3-pointer every time.)
Incidentally the data sheet for the aeroplane is online
here (about 400k).
G