My own personal preference is for the 642, but between your choices I'd go with the 638. I would also lean heavily towards a pre-lock version, or at least replace the internal lock with The Plug. I would probably also have it converted to DAO (I know...I'm weird that way). I always wanted a DAO Bodyguard series gun because I prefer DAO guns for self defense, but the exposed hammer nub makes it easier to check for high primers than with Centennials, and I like having the ability to holster a gun with my thumb on the hammer for a little extra safety margin. But because the hammer is shrouded it won't snag on anything when drawing in a hurry.
I will also echo the advice of getting two guns if at all possible. I recently re-acquired a 642ND, so now it serves as a back-up to my 642-1 in case of breakage or in the unlikely event that I have to use it in a self defense encounter, which would require the gun being taken by the police as evidence. Even better would be getting an equivalent 649, allowing you to carry the 638 with some occasional practice, and doing most of your practice with the more durable, albeit heavier, all-steel 649. And you can still use one to back-up the other if necessary.
Again, this is what I would do if I were in your position.
As for grips, any grips for round-butt J-frames should fit. Keep in mind that there will often be some variance in terms of manufacturing tolerances, so even with the same type of grips from the same manufacturer there may be some very minor differences in terms of fit.
People will recommend what they like and what works for them, but grips can be a very personal choice, so it would be best to try them out before buying, if at all possible. At the same time, if you try a set of grips that don't work out for you, you could probably sell them and recoup at least some of your money back.