If I did my math right.....
for healing
1 aim = .13 heal pwr
1 aim = .003 crit %
1 pwr = .17 heal pwr
full set of aim augs would 2.10% crit and 94.64 heal pwr
full set of pwr augs would be 0% crit and 123.76 heal pwr
now this may be wrong, but I figured this out by just taking a piece a gear with either a mod or enhancement and worked it. So depending if you need Crit or not that's the factor. I my opinion I go with main stat since I see a decrease in Crit %,