It might just be my personal thought, but I think Nino used the kanji 唄　instead of 歌 because he always seems to have some kind of humble aspect in his lyrics. The title itself is very humble, meaning that the song is "nothing special", it's just something you could "find anywhere". The kanji 唄 to me seems like more of just a little song that you sing, like a lullaby or traditional song, whereas 歌 can be more of a big production, so 唄 seems more humble and simple to me. Even though the song has a big, grand feel to it, he's saying that it's really just a simple song that he wrote, but since it's about life which can be both very grand and very simple at the same time, I think he is contrasting the music with the title using the kanji 唄... that's my theory, haha ;^^ Maybe I think too much into Nino's songs, but they are always so deep in various ways~

... and for solos my list is...
1. Nino~!!!!!!!
2. Aiba
3. Leader
4. MatsuJun
5. Sho-chan

Of course Nino's is the best, lol (biased much?), then I LOVE Aiba's solo, it's so cute! I love the jazzy music and the lyrics are adorable. It just exudes Aiba all over the place too, lol ;^^
Oh-chan's solo I like because I want to see him dance it! His past few dances have been very calm (although also very sexy~!) so I want to see his 激しい dancing XD
Then I like MatsuJun's 'cause it's super catchy :D And Sho-chan... nnnn.... Sho-chan~~~!!! The title had me expecting another "sexy" song, so when it turned out to be a cheesy pop-y song it kind of threw me off... and then... he doesn't sing it very well... though I still think it's cute... I can't say that I actually like it yet. It's starting to grow on me though, so I think I just have to listen to it more.

And now my comment is way too long, so I will stop talking ;^^