Is there any reason I shouldn’t replace my current SRAM PG1070 cassette and pc1091 chain with an XT cassette and XTR chain (Shimano’s over £15 cheaper and a little bit ligher)
Am I missing something here?

I would

No issue at all

It’s not like SRAMs harder wearing or anything, I’ve always been a Shimano fan and this is the first bike that’s fully SRAM, had their chains before and always thought they were rattly when they wear.