Glad you see "the light".
Are you sure your top cog is a 12 instead of 11?
What I would recommend, is that you closely observe the actual cogs you use (tooth count) and base your new choice on that.
I kind of like having 1 gear lower than what I ever expect to need. Kind of a bail out gear in case I need to "limp" home.
If your riding conditions tend to be flat terrain, pay attention to the cog you use the most for your "cruising". You'll want CLOSE (1 tooth) spacing in that part of the cassette. That allows you to adapt for slightly changing head/tail winds and maintain your cadence at your most efficient rate.

I have an old "commuterized" RockHopper with a 7 speed. I've gone to "customized gearing" on it by taking cogs from 2 different cassettes and mixing.
I had a 12-28 and purchased a 13-26.
(12-14-16-18-21-24-28) & (13-15-17-19-21-23-26)
I made a 13-14-15-16-17-21-26. (a 20T would work better than the 21T, but....)
I seldom used the 12 and the 28 was lower than I needed. (i use the 38T ring around town)
The 2 lower gears have pretty big gaps between them, and I have to "spin up" more than normal. Once I reach the 17, I have my close spacing that works well with my normal 14 or 15T cruising cog. IF I need higher gears than the 13T, I switch to the large ring and have 4 more closely spaced higher gears.
With your 9 speed, you have much better choices to accomplish the same thing without "jumping through hoops".
Just do a bit of research for what you want to accomplish before purchase.
Sheldon's gear calculator can help a great deal for "what if" scenarios.

http://sheldonbrown.com/gears/