Is the GA-B2100C-9A the same watch as the GA-B2100-1A?

Yes. They use the same module, case, crystal and strap material. The difference is the colorway: black versus yellow.

Which is better for everyday wear?

The black GA-B2100-1A. It matches more outfits and hides scratches and dirt better than the yellow.

Do both have solar charging and Bluetooth?

Yes. Both run on Tough Solar and pair with a phone over Bluetooth for time correction and phone finder.

Neither. Same case, crystal and solar cell, so wear and battery life are identical. Color fading on the yellow is the only long-term difference.

Is the yellow one worth stepping up to?

Only if you want the color. There is no hardware upgrade, so pick on looks alone.

Casio G-Shock GA-B2100 vs Casio G-Shock GA-B2100C-9A

These are the same watch. The GA-B2100-1A and GA-B2100C-9A share the module, the Carbon Core Guard case, Tough Solar, Bluetooth and 200 m water resistance. The only real difference is the colorway: black resin on the 1A, bright yellow on the C-9A. Buy the black if you want one watch for work and weekends; buy the yellow if you want it to be seen.

Choose the G-Shock GA-B2100-1A if you want the standard black colorway that goes with everything and hides scuffs.
Choose the GA-B2100C-9A if you want the yellow colorway and the same hardware underneath.

Prices

Casio G-Shock GA-B2100: $134.99 at Amazon (last seen Nov 9)
Casio G-Shock GA-B2100C-9A: $109.24 at Mercari (checked today)
Difference: the Casio G-Shock GA-B2100C-9A is $25.75 cheaper than the Casio G-Shock GA-B2100 at the latest prices (Nov 9 and Nov 10)