I think it's off sale by now, but maybe this will help some people decide in the future. I use Substance Designer to make textures for my personal builds in SL. I love it, but you should know things going into it:
It's a complicated program, much like Photoshop is. There will be a LOT of learning to do.
Much of the program is focused toward game makers and 3d artists who have .sbs support directly in their product, so not all of the features will be useful to you.
SL does not support .sbs subtance files, of course, so you'll have to export your textures to regular files.