I often use Bittrex as many people here, because their user interface is simple and has a good volume. I've been using it for about half a year now and never have I had any verification problem or wallet freezing problem.
Ethereum has more community support than Waves, more investors. I don’t think they can be compared. Ethereum sits very tightly in its niche, and Waves won't be able to compete with their platform which has so many functions and was there first.
It all depends on ICO: the better the ICO, the bigger chances it has to be successful. If the community is behind it, then you should buy in the ICO, and if the community support is not so good, then you should buy tokens or coins when they are in the market. No matter where you buy it, if it is a good coin, you won't regret investing in it.