If you're going to compare, you have to compare the spells in all aspects:

1. Basic healing spell (GCD)
Cure: 400 potency. 2s cast (200 potency per second), 2.5 recast (160 pps).
Benefic (Diurnal): 380 potency. 1.9s cast (200 pps), 2.37 recast (160 pps).
2. Big healing spell (GCD)
Cure II: 650 potency. 2s cast (325 pps), 2.5 recast (260 pps).
Benefic II (Diurnal): 620 potency. 1.9 cast (326 pps), 2.37 recast (261 pps).
3. Big healing spell (Off-GCD)
Tetragrammaton: 700 potency. 1 min.
Essential: 400-1000 potency. 40s.
Do the same math above for every GCD spell, and you'll see that the overall healing is the same on skills like Medica/Helios and Medica II/Aspected Helios.
The two spells that WHM has access to and that AST doesn't have are Assize and Benediction. AST needs to use Swiftcast+Helios to achieve the same effect, which is fine because we have more instant cast spells and we don't need Swiftcast that often.