Based on what I have heard from the shop that sold me my 12 M ION and other riders, the 48 cm bar is better for the 12.
The 54 cm bar does work ok though. In the end it is a matter of personal preference.
If you have not yet flown the kite, take a look at some of the tuning tip threads in this forum. In stock condition, the kite may stall or go backwards on you until you get your line lengths tuned.
http://community.flexifoil.com/showthread.php?t=98100