Cassette: Sram 991 or XT??

XT kassett. 11-32: 269gram,
Sram PG 990 kassett 11-32: 273gram

I think that XT performs better.

I've had the XT for over a year, took me through 1.5 winters and I rode London-Paris on it during the summer + all the training. Its just looking like it could do with replacing but has been super reliable and its a decent weight. If your looking for cheap + light weight (but with a bit of gearing lost), why not try 11-28 slx with a alu lockring? Comes in at around 222g

The best shifting I have ever used was XTR M970 Shadow RD, Sram PG990 Cassette and KMC X9 SL Ti chain sooooo smooth and quite too.

Why do you want to use a 10spd chain? The KMC X9-SL Ti is light, shifts well, and has lots of proponents.

The X10 is lighter and just as strong apparently.

Not used an XT, but I have just replaced my PG990 with an XTR (both 11-32). Not sure the XTR is worth twice the price. Never had a problem with the SRAM - got over 2,000 miles out of it - think it looks better than the XTR too.
I would imagine that the XT and PG990 perform very similarly. They also cost about the same, but the XT is slightly lighter - 20g or so.

Do the Ti cogs on the XTR wear out before the rest of the cassette or does it wear evenly since the middle steel cogs are used more and are smaller?

I have had both. They are very much the same. Where I live the XT is a bit less expensive. Also lighter. For these reasons I prefer the XT. The SRAM scores higher on the "looks" department.

+1 XT. In my experience it shifts better vs SRAM.