Jimbo, I am no expert, but the I think, the best thing to do is both....Swimming is one of the best exercises you can do, and it does work all the body systems and muscle groups, but it I do not think that the added resistance of water, will aid you in increasing muscle, unless you are swimming at the Age group or college level. Think 5,000-15,000 meters a day.
Most swim programs incorporate a weight workout/component to the program, although when I believe the weight workouts were more for power then for hypertrophy.
I know this probably did not help you. Hope you can find a happy medium..oh maybe not, if you go lift, then you are not racking up the meterage and keeping up with me..LOL