Best thread for sewing leather?

[HippieLee]

Is "Irish waxed linen thread" such as found here: http://www.royalwoodltd.com/cat14-17ar.htm
suitable and practical for leather projects? They have a wonderful array of colors but I want thread that will last and I simply don't know what it should be. Is the x-ply rating just the thickness? I'm assuming a 2 ply would be more suited to a smaller stitch, lighter item (say a fancy wallet) while a 3 or 4 or more ply would be better for holding together a sporran or knife sheath?

So far the only thing I've used is the brown and black waxed thread from Tandy. But that's the only colors they have (well, and a natural/white I guess). No idea what that's made of but I can tell the brown I used on my ID badge holder for work is already faded. Maybe that's a problem with all of them...

thanks!
HL

edit to add~ I mean for hand-stitching if that makes a difference...