I'd say most places are much of a muchness in general. How a shop is as much dependent on their manager of the day rather than any overall chain dynamic. Most are franchises I think.
I used to be a rep - my main tips would be really nice to the people working in the shop - and try and go when its not busy - which can be difficult. Below are my tips on renting skis.
To get maximum enjoyment out of skis (and the skiing on your holiday) and if you want to feel the difference, go for their top end rental level as this where the "proper" skis are generally found. Cheaper rental packages have skis that were purpose made for rental shops (super hard/heavy/slow bases and reinforced edges for maximum abuse by punters), whereas the most expensive package should have the actual skis you could buy from a shop. Also, ask if you can change once of even twice during the week.
When i'm renting, I pick my ski based on the conditions in resort and what I'll be doing. I'll often rent for a day or two even when I've got my own skis with me - if my skis aren't ideal for the conditions or I just fancy a change - I only get to go every so often, so an extra few quid on a days ski rental can be worth it.
If there is some powder about and you want to get in it, get something with around 100mm or more waist - it will make a huge difference. The free float provided by the ski can make up for a lack of technique.
An "all mountain ski" should have 90mm plus, but anything around the 90mm mark will still lack that effortless float, and you'll need better technique to counter that.
If there is no powder about, and you know you'll be sticking to the piste - experiment a bit. There is nothing quite like a day haring round the slopes on some 165cm slalom skis, but after a day of that your thighs will probably be done.
My general piste cruiser of choice would be about 80mm underfoot with a circa 20 meter turn radius. Great for hammering about in nice controlled arcs with tons of sidewall grip.
If you want to dick about jumping off stuff at the side of slopes and in the park, get some centrally mounted twintips. They will be softer in the nose and tail and loads lighter. This will help pop and the central mounting will help if you want to go backwards.
A nice shop might let you take out all of these on different days. A bit like bikes - there is a different ski for every different condition/slope/general persuasion. Trying to find the "one ski quiver" is the same as the unicorn of the "one bike to do it all".
I'm not going skiing this year and I think it shows in the length of my reply and my yearning for some snow:(