GDAX has a nice trading UI and you get familiar to it pretty fast, even if you didn't have traded before. The most important trading features are there: market order, limit orders and stop orders. There is no margin trading, though (You shouldn't do margin trading as a beginner anyway).
If you are already verified on CoinBase, no additional verification is required. Plus, you can transfer your fiat assets between CoinBase and GDAX instant and without any fees.
Kraken has had many technical issues and downtimes in the past. In the last months this has improved and trading was pretty stable. The UI looks outdated but it works. All trading features, including margin trading are available.
My experience with the Kraken support is pretty positive. I had an issue with a bank transfer (wrong reference) and the support was very helpful and solved the issue within 3 days.
Coinbase is good, as long as you don't need the support. My account balance showed up as negative because of a bug in their wallet. Because of the supposedly negative balance I was not able to withdraw any funds. It took CoinBase 7 months and 4 support tickets to finally solve the issue. Support tickets are getting ignored or replied by bots. It is very hard to get through to a human supporter.6 months ago
The process of buying / selling on Bitcoin.de is different than on other exchanges. After selecting an offer, you directly send a wire transfer to the seller. The downside is that real time trading is not possible. However, you can sometimes find very good deals.6 months ago