I’ll compare them by some of the most important features.
Both Poloniex and Coinbase have high trading volumes.
Number of trading pairs:
Poloniex offer 80+ tradeable coins, and the majority has good liquidity. Coinbase offers Bitcoin and Ethereum trading.
Poloniex offers margin trading in more than 10 altcoins. Coinbase doesn’t offer margin trading.
Poloniex: 0–0.25 percent
Coinbase: 1 percent fixed
Percentage for other transactions such as deposits and withdrawals by different payment methods varies but Poloniex is always under 0.5 percent and Coinbase is always above 1 percent (in some cases even close to 4 percent).
Both have really good security, but Poloniex was hacked in 2014. Coinbase was never hacked.
Poloniex has relatively poor support. Coinbase support is better, despite the use of “canned” responses.
Poloniex doesn’t support fiat trading. This means you can only trade cryptocurrencies. It also suffers from often pumps and dumps, which means that the whales tend to influence the market, especially in less popular coins.
Coinbase is known for putting investors before their customers. They are also known for major violations of user privacy, which brought them the bad name.
That’s all I have for now. I hope these comparisons give you a bit more insight about the advantages and disadvantages of these two trading platforms. I personally prefer Poloniex, but you should read some online reviews and make your own decision.