Are all 4-arm 9spd XT chain rings compatible? I ask this because I notice different prices for what look like the same rings. Or is it worth paying more for Middleburn rings in the hope they will last longer?

depends on the year of your cranks. From the M770 onwards they are all compatible. But middle rings from M770 onwards won’t fit earlier cranks properly due to the composite bit. Unless you get the file out.

SLX will fit M770 onwards

Latest 10 speed deore will too as they are the newer style.