There are a couple of different schools of thought on this question.
If you get a bigger hitch, you may be under the hitch's weight rating without a WD system. However, you must be sure that you are not exceeding the weight rating of any other part of the truck--most significantly, the rear axle. A WD system transfers tongue weight from the rear axle to the front. It may be the case that your rear axle is overloaded without a WD system, but safe with one. Realistically, however, this is a bit far-fetched, as every truck I have had has had a rear axle weight rating that would not be exceeded as long as you stayed underneath your vehicle's GVWR. And, furthermore, few people really bother to determine exactly HOW much weight the WD system is transferring from the rear to the front. (If you cared to, you could do this by weighing the truck's rear axle and front axle with the WD bars in place, and then with them removed, and calculating the difference.) So, realistically speaking, I don't think that the rear axle's weight rating is likely to matter--although you should check just to be sure.
What's that leave? Well, this is a trickier question. I have heard some argue that the vehicle itself is given the maximum tongue weight as a system. So, you think about the tongue weight as being relevant to the hitch itself. You replace the hitch, you get a higher tongue weight? But if you look at your owner's manual, it probably doesn't say, "Your hitch is rated X." It says, "You can tow X." And my factory hitch doesn't even have a weight rating placard or stamp mark on it. I just have to go to my owner's manual. So, some have argued that it doesn't matter if you put a bigger hitch on, because the manufacturer has not certified the vehicle in that configuration, and as far as the DOT is concerned, your vehicle is rated to tow whatever the manufacturer says and that's it. I don't give that argument much credence, because if it was true, then the entire aftermarket hitch industry would be out of business. I assume that the hitch manufacturers have done sufficient testing to ensure that my vehicle can safely use their hitch, so if I get a bigger hitch, I would assume that I'm good to go, as long as I respected all the other stats (GVWR, GCWR, GAWR, and so forth).
But even if I didn't need a WD system to tow my trailer, if I was towing a significant weight behind me--say, more than 2500-3500 lbs--I would opt for the WD system, because it makes the ride so much better. The vehicle and trailer move as a unit, and it doesn't feel like the trailer is pushing the vehicle around or "diving" during stops. It's really nice.
If you get the bigger hitch, try it yourself both ways and see if you notice a difference.
