mtb cassettes-sram or shimano ?

will be changing the cassette on my mtb in a week or so which has a sram pg750 on at the moment,which would be a better replacement-pg770/780 or an equivalent shimano and which shimano group would you recommend ?
boardman mtb 2008
boardman road 2011
nice bit of kit
boardman road 2011
nice bit of kit
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boardman road 2011
nice bit of kit