so with a 'proper' 2x10 chainset, you can use all of the cassette in either ring, whereas on a 3x10 with the outer taken off, you shouldn't be using the smallest end of the block when you're in the granny..
But you can, and it makes sod all difference, having used M970s and Lightning cranks converted to doubles and XX and M985 'pukka' doubles.
It's about the gears you use and what works for you. I found a 22t too small to be of use, and a 44t too big, so went double, on a 2x10 I had 28/42. A lot of people find no use for the 44, but like the 22 as a bail out, and thus like 22/36, often with a bash.
I personally found myself in the 42 the vast majority of the time, but right up the top of the block, ie crossed over, so I've changed to 1x10 with a 36t ring and find that spot on. A lot of people do it for weight saving or to reduce clutter on the bars, where as personally I just found the gearing what I was after, with those as happy by-products!
If you're going for 10 speed get an 11-36 cassette, otherwise 10 speed is pointless, stick with 9.
Basically... work out what gears you use and what you don't, then think about what would suit you best.