Neither.
I'll do some number crunching (and I hope I didn't miscalculate anywhere, feel free to check my calculations, fellow number crunchers) for you to highlight why it makes sense for an Ast to hardcast rather than the WhM since this is the comp your kills usually had without it being considered "carrying".
Asp. Benefic and Regen both have 1200 potency in total, Asp. Helios and Medica II both 700 and Bene II and Cure II 700. The only difference is that Asp. Benefic has a small upfront heal but shorter duration than Regen.
Ast ST target hardcast have a total potency of
2120 for Asp. Benefic through Neutral & Synastry
1120 for Bene II through Neutral & Synastry
WhM ST hardcasts total for:
1440 for Regen with Temperance
840 for Cure II with Temperance
That means Ast heals for an additional 680 potency through Asp. Benefic over Regen and 280 through Bene II over Cure II.
Ast AoE hardcast heal totals for:
1540 with 400 of it being on demand for the next 30sec through Neutral & Horoscope
WhM AoE hardcast totals for:
1040 from Temperance & PI
Meaning Ast heals an additional 500 potency.
Ast heals for 30%-47% more with one hardcast. Unless they already have trouble with their MP, there is no good reason for a WhM to hardcast with both tools up over an Ast hardcasting with both tools up, escpecially since WhM loses 50 more potency for that GCD heal over Ast.
Immediately healing every AoE with Med II or Asp. Helios is not carrying. It's wasting MP while snipping the other healer's regens and that is poor gameplay.
PUGs cannot be expected to be well-coordinated though I generally had a good experience with communcating which AoEs and busters I can cover without help, cutting down additional healing to a minimum. But if I see my co healer being really aggressive about healing and immediately casting AoE heals, I will naturally cut back on my own healing, even on oGCDs. This isn't a HpS dick fight. In those cases I'd rather save my tools for times when my co healer panics, messes up, is out of MP, died, the raid as a whole messed up etc.
e5-e7 can be cleared without a single hardcast aside from transitions, although I wouldn't expect that to happen in PUGs or more chaotic statics, because properly spread mitigation and oGCD heals is the exception rather than the rule and I will not risk a wipe because "I shouldn't have to hardcast here!" - I don't like it, I know it can be avoided but I'm definitely not above it if it gets us the kill.
Unless my WhM/ Sch co healer is a heal bot, I will cover the neccessary hardcasts out of my own violation as much as possible because I know it costs them more for less gain than me.
And since I'm not running a low Piety set in PUGs, I can manage my MP even with the additional hardcasts. I'm not carrying anyone doing this.
And in a decently coordinated group I can absolutely get away with not using a single hardcast. I have this happen regulary on Ramuh and Ifruda (not counting transition hardcasts ofc); only on e7s I tend to hardcast during uptime here and there because it depends on kill speed what I have up for Blasphemy waves, 2nd portals need some decent mitigation and people still tend to spread out too much for colors + waves during birdnado to cover all of them with Star & CU.