1)
Why should Yerushalayim praise?
1.
Radak: Hashem desires you, for you trust in Him. "Yerushalayim" refers to its residents.
2.
Malbim: Shevach is making something better than it was. You (Yerushalayim) increase Hashem's praise more than it was via all the nations.
2)
Why does it add "Haleli Elokayich Tziyon"?
1.
Radak: The matter is repeated in different words.
2.
Malbim: "Hashem" refers to the general conduct that the Creator established. "Elokayich" teaches about His connection to Yisrael, to do for them wonders. The Chachamim and officers were in Tziyon; they praise for the wondrous Hashgachah. The commoners were in Yerushalayim; they praise for the general conduct.
