I choose for all capitals because it's katakana. It seems the title is in the same style as Koharu's "PAPANKEEKI". It doesn't mean anything by itself, but there is a foreign word somewhere in there.
Actually, katakana isn't just used for foreign words. It's also used for most sound effect words/onomatopoeia, and "Tantantaan!" is more likely to be onomatopoeia than a foreign word. Maybe they will use their tambourines again in this song and "Tantantaan" is the sound of them playing the tambourines.
I liked Anataboshi, and I like Kikkawa Yuu a lot and think she blends well with Koharu and Sayaka (and her solo version of Anataboshi was the best), so I'm glad her unit is getting another single.