I was just wondering which organization is better for competing in the U.S.A. and why?

neither it really all comes down to the individual competitions.

as an amateur competitor, what are the most important competitions to attend?

If you want to go to an IDSF (the world governing body for "amateurs") World Championship, then you need to dance in at least one USA Dance Regional competition and also in Nationals. So, in that sense, the USA Dance competitions are very important to attend. There are six Regionals per year, the three biggest tend to be (in order) the Northeast, the Southwest, and the Northwest.

Otherwise, it all depends on where you are and what the local USA Dance clubs are doing. Most people participate in both USA Dance and NDCA competitions. Some NDCA comps are really awful for amateurs, and some are really good. Where I live, most amateurs make a point of going to the Emerald Ball and the Ohio Star Ball. A lot also go go the USDSC (United States DanceSport Championships, not to be confused with the USA Dance National DanceSport Championships).

It used to be that your USA Dance membership was accepted for entry into NDCA competitions, but a few years ago the two organizations had a spat that really never made any sense (especially in light of what each has been doing since then), and after that the NDCA started requiring its own registration card for amateurs dancing in their events.

I was just wondering how many people here compete in either division (if at all) and why?

I prefer USABDA because there is a lot of competition at our level. NDCA has few amatuers.

We will travel for usually 3 USABDA regional comps or 2 and a national each year and then attend NDCA that are more local.