If this is a rig that's going to see significant amounts of pavement time, and you're uncomfortable with the amount of body roll, I'd HIGHLY advise going to an anti-sway bar set up. Yes, I know it's a pain in the butt to deal with going on/off a trail (which is why I ditched both of them on my Grand
), but it's going to result in a MUCH better set up on the pavement than going to a stiffer spring set up that will also negatively affect you on the trail.
Spring rate should be chosen based on the weight of the vehicle that they're supporting, and to a lesser degree, ride height (ride height can be adjusted in other ways as well that will not affect spring rate/ride comfort). Yes, this is largely a trial and error guessing game for the average guy, as most of us don't have a good way of measuring unsprung weight of our rigs.
Also keep in mind that adding leafs will not just stiffen up the suspension, but will also raise your ride height as well as your center of gravity, which can also have negative effects both on and off road. This is also where a lot of the "cheap" lift kits suffer, and get reports of cruddy ride AND cruddy performance off road - the companies just throw in a much stiffer spring which jacks up the height and doesn't allow the suspension to fully cycle off road.
Where as adding an anti-sway bar will do none of the above, will only help to keep body roll in check, and still allow for full suspension movement when disconnected.