No; swimming is better for you, a LOT better. The reason is simply because swimming requires you to keep whole body moving to prevent drowning (and then some!), therefore toning your whole body - particularly legs and shoulders/upper arms. Naturally, it'll help you lose weight - but, yes, you will eventually develop muscle.
Gym activities - like, say, running - are of course good for you and will help you to lose weight when intertwined with a good diet, but they will only tone certain parts of your body. I've heard that the two sports that will kick you into shape the best are swimming and trampolining, so taking those two activities up would be a good idea if you want to get fit/lose weight (or do both at the same time, maybe?).
And, yes, I think you'll be fine and dandy with the gym-twice pool-twice regime. I highly doubt it'll make any difference if you do the two activities in a different order. Good luck with losing that weight!