T-55A (208 points) is cheaper than T-55 (211 points).
From the manual:
"The T-55A featured a number of NBC protection additions and changes. The tank's machine
gun arrangement was changed, and the coaxial machine gun model switched.
In Combat Mission, the T-55A replaces the 100 mm BM-8 APDS with 3BM25 Armor-Piercing
APFSDS."
So it is clearly better, yet costs less.