What explains 9 speed cassettes price differences?
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!