Personally I would stick with the A55. The advantages offered by the SLT technology far outweigh the A390. In this case, the A55 also offers superior image quality and high iso performance. The ONLY reason to go for the A390 is if you prefer an optical viewfinder, although I believe the A55 EVF is superior to the substandard OVF in the 390.
Also careful when you buy batteries. ONLY buy SONY batteries, after-market versions are NOT supported. only info-lithium will work. I have tried 3 different after-market batteries. Only one was recognized by my A850 and after 3 months it will not hold a charge. Make sure you buy only Sony batteries.
Sony A850 | Sony Battery Grip | Carl Zeiss 24-70 Vario-Sonnar T f/2.8 | Sony 70-200G f/2.8 SSM | Lensbaby Composer Pro | Sony HVL-F58AM
Alienbees B800 (x3) | Alienbees Cyber Commander | CSR+ Receivers | Grids, Softbox, Gels, Manfrotto/Century/Avenger stands