I run XTR shifters and have an XTR cassette which is due for replacement.

So I'm now looking at my alternatives for a 9-speed cassette – XTR, XT or a SRAM PG990…the SRAM looks like a good cassette reduced to fifty quid against 100 for an XTR.

How does the SRAM compare and are their any issues with using SRAM in a Shimano set-up?

Nope, will work like a charm!

It#ll work, but I believe the sram cassette will be heavier.

Yes, I thought the PG990 may be a bit lighter but at 305g for 11-34 it appears to be 50g heavier than the XT (and around the same price!)

The 11-34 xt weighs 297. There is a difference of 8g.

http://www.performancebike.com/bikes/Product_10052_10551_1032052_-1___