The 11-23 will make the jumps between gears a little smaller than a 12-25 making it more likely that you will be in an ideal cadence.
If you are using a standard large chainring
(52 or 53
) you may want to consider a 12-21 or 12-23. The 11 will only be useful if you are spinning out on the top end and most people do not have that problem unless they are going downhill.
Below is the gearing for the 9 speed Ultegra
(11-21T
) 11, 12, 13, 14, 15, 16, 17, 19, 21
(11-23T
) 11, 12, 13, 14, 15, 17, 19, 21, 23
(12-21T
) 12, 13, 14, 15, 16, 17, 18, 19, 21
(12-23T
) 12, 13, 14, 15, 16, 17, 19, 21, 23
(12-25T
) 12, 13, 14, 15, 17, 19, 21, 23, 25
(12-27T
) 12, 13, 14, 15, 17, 19, 21, 24, 27