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There is a series of very in depth podcasts about this from Blister. They chat with one of the team from Atomic who describes how they balance the first fit feeling with actual comfort after hours of use. I think rental boot liners are often padded out with thicker, softer foam which makes them feel comfy when they are new and you first put them on, but then eventually they pack out, your foot slips around and they just feel dogshit.
Bring your boots if you have a good pair but rent skis. I’ve had rental boots and couldn’t find a decent pair even though I tried half the shop on. I’m not sure but I got the feeling they have certain rental models that are comfy in store but shit on the slope