The argument here seems to be balance vs power, where one choice is better for the player, the other choice is better balanced, and presumably intended.
However, there is a comment from Mike Mearls that, yes, it can give a higher total than a non-durable HD roll (this comment has been mentioned multiple times above). This comment suggests that the less-balanced or powerful way to play this feat is indeed what was intended.
Also, I think people are overlooking the sacrifice of a feat. Feats are, for the most part, restrictive in giving stat points, or don't give stat points. This means that you're sacrificing the ability to customise your stat scores directly for the feat benefit - as with all feats.
So let's look at the balance. This is a non-combat trait, that won't always be in effect (not every day will contain a short rest), that will give no lingering affects into combat or other roll-based encounters (conversations or skill-based puzzles). It is sacrificing extra potential flat health, or damage, or additions to other skills. Having an extra Constitution score, or indeed +2 scores in other stats, is the sacrifice.
This also assumes that people are rocking a high Constitution. Many people with lower HD have higher scores in other stats. Examples:
HD6, Maximum roll with Constitution score of 16 (+3) = 9. Minimum Feat roll = 9. Constitution needed to exceed previous minimum = 18
Cleric, Druid, Monk, Warlock:
HD8, Maximum roll with Constitution score of 18 (+4) = 12. Minimum Feat roll = 12. Constitution needed to exceed previous minimum = 20
Artificer, Blood Hunter, Fighter, Paladin, Ranger:
HD10, Maximum roll with Constitution score of 20 (+5) = 15. Minimum Feat roll = 15. Constitution needed to exceed previous minimum = 22
HD12, Maximum roll with Constitution score of 20 (+5) = 17. Minimum Feat roll = 15. Constitution needed to exceed previous minimum = 24
This comes back to stat score sacrifice - EVERYONE can benefit from constitution, but also dexterity, and wisdom. This means, if a wizard is pushing for 18 or 20 constitution, they are losing out on the potential for dexterity saving throws, or perception (wisdom) checks, or even maybe their intelligence spellcasting modifier/damage. This way Feats sacrifice flat stat score increases means that while some are more powerful than others, all of them still fit within a loose frame of balance. And while it is possible to get higher than 20 with items, that's no guarantee, and not as much within player control.
As someone who recently looked at this feat for a character, I spent a solid half hour determining whether or not it was better. Looking at the feat under the powerful iteration, I decided to NOT take it. I opted against because, even with the potential for healing above my possible HD roll (cleric with a constitution of 19 and a HD of 8) it wasn't as good as some other flat stat increases.