I just think embers are better in tanks as small as 10G. In a larger tank, it's probably not so much an issue. :) I have heard of nippy neons, but again, the person was keeping them in a small tank so it's not that surprising. In a 29, it's up to you. Personally, I'd get both. They are both gorgeous. :) Internet pictures don't really do embers justice, but neons always look amazing.
If it was a male betta, I'd get embers over neons because embers are so chilled, there is no chance of their swimming stressing a male out.