It shouldn't really matter. Though cooking in a pan can have an effect on surface texture of the bottom of the chicken (or not, if cooking on a grate), differences in tenderness or juiciness would come more from differences in birds (age, variety, processing) or finish temp (one is more cooked than the other) than cooking method, since both are dry heat.
I cook chickens (as a rule, between 400 and 500F) in both my kettle and my oven (in the oven I don't always use a pan) and don't find differences in finish.
Nor would I expect to find much difference between a chicken cooked at 400 and another at 500 (and I don't), whether one was cooked in the kettle, the other in the oven, or both were cooked in the same cooker. I might expect to find differences were one chicken cooked at, say, 250 and another at 350--but this has more to do with the perceptual difference of the softer skin of the one cooked low (and soft skin, being moist, can leave the surface of the breast meat more moist seeming as well). I'd also expect to find differences between a chicken cooked and rested upright and one cooked upright and rested breast down. In all theses cases, though, it shouldn't matter if the chicken was cooked in an oven, kettle or WSM.