1. Similarity to running:
Alter G: Very similar. Just like a regular treadmill.
Pool: You can mimic the motions, but without push-off and rebound, it's not very like running.
2. Effort required:
Alter G: Depends on the %weight setting, but obviously, there is less and less effort as you decrease weight. You feel like you're running, but not working out.
Pool: I've finally figured out how to get more out of pool running, so I'd say more effort required here. You're pushing against water, so that adds resistance.
3. Impact on body:
Alter G: Although this is dependent on your weight setting, your foot still strikes on the AlterG. There are plenty of injuries that I don't think it would be safe to rehab on the Alter G, at least not initially, like a broken foot.
Pool: No impact if you're deep-water running. Probably the safest rehab exercise, although you can stress soft tissue by pushing against water resistance. When I'm pool running after an injury, I eventually transition to running on the floor of the pool, and I would say that amount of impact is comparable to what I feel on the Alter G at 50% body weight.
4. Place in training:
Alter G: Because you can adjust the impact, you can use the Alter G anywhere along the rehab continuum, all the way from early rehab to back to racing. However, for most of us the Alter G is only appropriate for rehab, not regular training, since a. no one has regular access to one and b. there is so little effort that you are not really "exercising" - and unless you are a pro runner, I'm sure your time available to dedicate to running is limited and you don't want to waste hours without getting some cardiovascular/endurance benefit from it.
Pool: Although there are only two levels - deep water (no foot strike) or shallow water (foot strike), you can still use pool running in a variety of rehab and training scenarios. Actually, pool running is good cross-training, and I keep saying that I'll keep it in my schedule once I'm back running. I probably won't because I'm lazy, but it would be a low-impact alternative to running.