IMO the spyderco doublestuff is probably the best pocket stone currently available. That said, my go-to is the ceramic half of a very old dc4 and a DMT diamond "stone" (the red one). I only use the DMT because the diamond wore out on my dc4 and I don't like the "new" style ceramic that fallkniven use. If I was starting from scratch I'd get a couple of DMT stones in different grits, and maybe try and find a nice smooth piece of ceramic from some old hair straighteners, the only reason I wouldn't get the spyderco is that they're a bit expensive to my mind. The "best" option is probably a few different grades of wet and dry glued to pieces of plywood, they stay flat, they're always "fresh", you can make them whatever size you want, having many different grits is always going to give a better edge than having just two...however, you need to look after them a bit more and they don't look quite so gucci.
tl:dr, stick with the spyderco, you've already spent the money and they're really very good.