I would suggest with the XPERIA X10i, you have to look at the complete package.
You're going to need a good Class6/10 microSD card and with prices becoming lower all the time, I would suggest nothing less than 32GB.
You'll need a good battery. I have chosen a Mugen 1800mAh
No matter what ROM/Kernel you're running you'll need to install Link2SD.
This complements two factors:
(1) Frees up the /data storage space
(2) Gives you the benefits of high-speed read/write.
I use the following ROM/Kernel
ROM: GB 2.3.4
Kernel: DooMKernel V6
Battery Life 1-3 days normal use (7-10 days in standby)
Benchmark: GB 2.3.4 and DooMKernel v6
I don't know what the original ROM was that I install, or if I just installed bespoke applications and libraries.
I do have Sony Weather Widget, swipe to clear notifications, screen snapshot, XLoad and Braiva Engine.
If you're going to watch 720p videos or stream TV to your X10i, you'll need a GB v2.3 derivative ROM as it's the only one that'll support GPU HW acceleration for 2D graphics.