What explains 9 speed cassettes price differences? I have a pretty heavy bike I use to travel long distances with a lot of luggage. It  has a mix of XT, deore and other Shimano equipment. I bought it second hand so I don't really know the details. I have ridden 10,000km with it without replacing anything in the drive train, and it's now so used up the chain skips when I apply too much force on the pedals, and my foot just goes straight to the ground.

I want to change the 11-34 cassette and the chain to solve that. On the website I will order from, they have: 


a 14€ HG400 Alivio Sora
a 16€ HG201 Acera
a 47€ M770 XT


I have no idea what explains the price difference. As far as I can tell they are all compatible with my XT derailleur but the XT one is lighter. I don't really care about weight as I use this bike to transport heavy equipment anyway, but I do care about durability. 

What should I buy? Should I also change the rings at the same time? 

Thanks a lot for your help.
In most cases with bicycle parts, more expensive means lighter. There can be exceptions, especially when you get to the super low end parts where cheaper can also mean cheaper construction.

Since you put so many miles on your bike and it seems important to you, I would spend the extra 2 pounds and grab the Acera. XT is generally used on mountain bikes where weight matters.

Most importantly: Absolutely replace your chainrings! If your cassette and chain are that far gone, your chainrings are as well. You will get terrible chain suck the minute you put a new chain on. 

You can avoid needing to replace the cassette and chainrings by periodically replacing the chain before it gets too worn. Invest in a chain wear checker, they are cheap. Replacing your chain periodically will save you a lot of money!