The most important factor in maintaining good cognition is not using too much cannabis, regardless of strain. It's best to use just enough weed to achieve pain relief, and then put it away. Also, use a vaporizer to avoid toxic byproducts of combustion, with the added bonus being that many anecdotal reports indicate vaporizers produce a high that is more "clear headed" than smoking.
As for the strain, I think its best to just ask the budtender to show you some of their current high-end stock. Ask them which they personally think is the best strain in stock at the moment, and then compare that strain to some others. At the end of the day, you can't go wrong if you buy a potent strain with a nice appearance and a great smell. I know that's vague advice, but most dispensary talk is BS, and the variation between strains, and even between sativa and indica buds, is pretty small. Any potent strain will give you relief, especially if your tolerance is pretty low. After some experimentation, you may find that you get more pain relief from consuming certain strains versus others, but its just a small difference, and its more just a matter of personal preference.
An imperfect analogy would be to cigarette brands -- all of them will work, and essentially do the same thing, but some people intensely prefer one brand over others -- because minor variations such as flavor, cigarette length, density of the smoke, etc etc are very important parts of the overall effect due to psychological associations. Of course this analogy is not rock solid, because cannabis strains vary not only in flavor but in their proportional content of a complex mixture of pharmacologically active terpenes. But like I said, the variations will be minor between any sufficiently potent strains, because all will contain a bunch of CBD and THC, which are the main therapeutic components of cannabis (at least the main ones with any good research to back up their efficacy).
I feel like I've rambled a bit.. I'm high right now myself. :D