I find Urbans to be much deadlier than the Spartans, unless you are playing money game of course, but that's out of discussion.
Urbans are much cheaper, and they are so cheap for their effectiveness it hurts.
Urbans are good all around, Spartans while being good swordsmen, are still primarily a phalanx unit.
Urbans have pila, and are very resistant to missiles, Spartans have less armor therefore more vulnerable to missiles, but then in particular instances they are vulnerable to AP troops, but that's a lot more circumstantial.
Urbans are just all around better troops for their price. You can safely say urbans are the best infantry in the game. Well can't complain much since Rome dominated the infantry world in their time. I only complain about their cavalry being superior, which is something they did not have.
Patience is a virtue.