Greetings. You're right, those are the 1st generation in 42.2mm and the second generation in 41.5. The first one was a gift from my wife, and it was my first good watch. I recall there was an ad in a printed magazine and I couldn't stop staring at it! When my wife suggested a good watch should come from her as a gift on a special occasion there was no question it would be the Aqua Terra. When Omega updated the model with Caliber 8500 ten years later I began to squirm a bit about wanting to upgrade a watch that had been a gift from my wife. As another special occasion was approaching we happened to be in a nice jeweler's shop together, and this was around the time the blue dial was first introduced in the second generation watch. She loved it, I suggested the upgrade (as a gift from her again) would be a great way to mark the upcoming special occasion, and she agreed!
Pros for the first generation watch: white gold furniture under the crystal and slightly slimmer profile. Cons for that watch: legibility was okay, but there were times when the hands were invisible against the gorgeous but somewhat glossy black dial. Also, the Co-Axial components in Cal. 2500 are prone to go off in a way that affects timekeeping and requiring service from a qualified watchmaker.
Pros for the second generation watch: uh, I wouldn't change a thing. The date display can be a bit difficult to see from an angle because of the frame, but that's pretty much it. Caliber 8500 is fantastic, and in my opinion makes this watch undoubtedly the better choice over its predecessor. Proper, ground-up integration of the Co-Axial technology, twin barrels, modern materials, and beautiful finishing. I think Omega caught the competition off guard with a new industry standard with Cal. 8500 (look at what Rolex, JLC, Grand Seiko did a few years later). The case was tightened and toned a bit, and the bracelet and clasp were modernized, but the new caliber is really the big story, and I think it puts this watch in a different class than its predecessor.
Between the two I can strongly recommend the Cal. 8500 watch.