IMHO new character > constellations every time, but it would depend how much you want Eula. If you don’t want her, saving for Fontaine is a smart move at this point in time.

I personally value more characters, because you get a lot more variety and fun from having multiple characters then a slightly more powerful one imo. Also, not like this game is hard, and c0 Kazuha will easily clear all future content lol.

You should prioritize whatever leads to you having the most fun. Are you planning to use Eula on a team? If not, then it’d be probably be a better idea to invest in your main.