I read this post at work and I just stepped back, opened my box, looked at the one I use for seats, read it, and posted it lol. I hardly go by measurements, I just look for the socket I use for a particular application and use it.
screw snap on. imo not worth the money unless your a proffessional who is going to use the tools on a daily basis. that 10mm tripple square from snap on will cost you around $35 or more. just go on amazon and search it. you can get a whole set for that price.
Well, it's a good thing I haven't purchased the triple square set that I mentioned above.....cause I've just found an even better set which does include an 18. Here's the link: http://www.tooltopia.com/vim-xzns1000.aspx
For a diyer get the cheapest set you can find. I have a few different sets which vary in size depending what I need it for. I broke two bits in my life from the sets I have.... 6mm and 10mm. They get hard abuse daily from my impact gun.