You've really excelled yourself at these terns, the bouncing flight of them zipping around is wonderfully captured, and there's so much joy in them - just from looking at the sketches I can tell you were smiling at them.
The painting is lovely, I like the way the bird fills its frame in a natural way with gorgeous tones and colours. As you said that you weren't happy with the wing, here's what I think is the problem, there doesn't seem to be the greater coverts, which would account for that strong line that Tim mentioned. Now, remember I wasn't watching these birds (haven't seen any of these delightful birds for 7 years now) so I could be way off the mark. What you've painted could be right, but just looks odd. There was a thread on this somewhere in the art forum a few months back.
Another set of excellent work.
Oh, as for gouache v acrylic, the best is to have a play around. Acrylic dries waterproof and can be painted over without making it dirty (my overworked acrylics are dirty because I'm too lazy to change the water and to wash brushes and palettes), gouache, I find is a lot more 'slippery'. As you can umagine, it's imossible to really describe the properties of paint, as everybody uses paint in their own way and achieves different results. Have a play around and see what suits you.