I've used Arnold Clark on my last two trips to the UK. In 2023, I rented from them through their affiliate, Celtic Legend (14 day rental) for my trip to Scotland, and 2024 when I spent 6 weeks in England, I rented directly with AC (18 day rental). Both rental experiences went very well. Arnold Clark is a pleasure to deal with, no matter who you book with.
I would check with both to see what price they give you. Celtic Legend will sometimes offer specials if you book through them, like giving you free sat nav or a second driver for free. I usually don't take advantage of the sat nav because I prefer using Google Maps on my phone, and since I'm a solo traveler, I don't really need an extra driver. But it does pay to check there
The best thing about them is that when you pick up the car, the agents explain everything to you and make sure that you understand the fine print. They are good about upgrading sometimes as well (once I was upgraded to a Mercedes at no cost, and another time I received an automatica at no extra cost (I had booked a manual). They are always very courteous and helpful, and I will continue to use them in the UK for future rentals.
Arnold Clark has offices both at Edinburgh Airport and also one that's closer into town on Sighthill. I returned my car to the Sighthill location (I booked it in Glasgow) and it's pretty easy to find, but you can also pick up and drop off at the airport if you do decide to go with them.
ETA: With regards to Salzburg, I usually go through AutoEurope when I am booking in Germany or Austria or some of the other European countries. They will show you all the prices available for the various rental companies and make sure that your rights are protected. I have never had a problem with Auto Europe, and I've been booking with them off and on for almost thirty years now.
https://www.arnoldclarkrental.com/
https://www.celticlegend.co.uk/car-rental/
https://www.autoeurope.com/