totally different. lol.
Koichi is more
KinKi Kids, than Tsuyoshi is. Tsuyoshi has always been somewhat experimental with his solo stuff. Koichi's solo material is somewhat more theatrical...it's quite similar to 99%LIBERTY, Bonnie Butterfly, Yakusoku, etc.
my favorite Koichi solo is "Tsukiyo no Monogatari" found on KinKi Kids' F Album, but his (only) solo album "mirror" is great as well.