This will give you the delta between sum1 and sum2, expressed as a percentage relative to sum2:
select sum1 = s.sum1 ,
sum2 = s.sum2 ,
delta = sum1 - sum2 ,
delta_pct = 100.0 * ( sum1 - sum2 ) / sum2
from ( select sum1 = sum(case when t.c1='a' and t.c2='b' and t.c3='c' then 1 else 0 end) ,
sum2 = sum(case when t.c1='x' and t.c2='y' and t.c3='z' then 1 else 0 end)
from table1 t
) s
using the derived table in the from clause makes for cleaner, more readable SQL as you're not duplicating the aggregation expressions all over the place.