Camelbak or Platapuss every time.
I can remember a few years ago, when unwisely i thought of walking the Carneddau from Cwm Eigiau, in what proved to be nearly 29C heat.
I carried my extra large capacity Camelbak bladder, but even then i must have drunk well over five litres of water, after refilling it in Cwm Caseg, i just could not get the fluid down me quick enough.
In hind sight i should have turned around and gone home, but apart from the heat, it was a lovely day for a walk, no wind, just a fiery furnace.
If you know you are likely to experience very high temperatures, now may be the time to invest in a new Camelbak sack, at least then you will be fully prepared.
If you do have to go walking in very hot conditions, which can not be unavoided, at least go fully prepared.
There's no point in having to curtail a walk, because you have no got enough water.