Other things we considered were the types of music available in the community- if the band director is telling you the orchestra is strong, I think that is important, and might look more closely at orchestral instruments if you think your DS will want to play in school.
We also considered the versatility of various instruments- the string bass is great because one can play jazz as well as classical; the kids who play bass here have tons of opportunities and flexibility. To be honest, the competitiveness of violinists here can be quite off putting- many begin when they are toddlers and I think it would be hard to catch up; perhaps not an issue if one is happy being part of a large section.
Just a thought on winds- our school also "waits" to begin winds in fifth grade (funny, we started in third grade back in the day...). Both my kids began earlier on strings, learned a good foundation and very basic proficiency, and moved to winds when they got to fifth grade. They chose instruments they liked- hopefully your school gives some sort of presentation/tryout the instruments period? Our kids' choices ended up suiting them well. I will add, though, that if your kids get really into music, they are not wedded to any one choice. It is common here for the horn players to take up trumpet so they can more easily play in jazz ensembles, pep band, marching, etc.; bassoonists generally learn sax very easily for the same reasons. And as noted above, many if not most music kids play at least some piano. I do think the piano has a really different focus, though; while a few kids do play in jazz or other ensembles, it is generally a much more solitary instrument. So I think it really depends on your/your child's goals for learning an instrument.
And yes to singing- if your kids enjoys it, it's fun, versatile, and there are so many ways to take part. I would think your school has a chorus? If not, there are community choirs, and many churches with choir are welcoming.
