If by "trading" you mean active trading, there are several factors - liquidity, API availability/quality, fee structure and the exchange having the coins you want to trade.

GDAX has a reasonable API, is very liquid, only lets you trade three coins for now (BTC, ETH, LTC). Fees are 0.25% for taker / 0% for maker orders.

Gemini - horrible API, two coins only. 0.25% fees. Not a good trading exchange. Very good as a "gateway" to move fiat in though. Wire transfers clear in a bit more than an hour.

Binance - lots of coins, API not great but ok if you're writing your own code, 0.1% fees. Fairly liquid.

Bittrex - lots of coins, ok liquidity, fees 0.25%. API - for streaming market data there is no published API and reverse-engineering it is a huge pain.

Bitfinex - lots of coins, 0.1% maker/0.2% taker fees. Not sure about API. Does NOT allow US clients.

etc. etc.

I didn't mention the web interface as a factor in choosing an exchange to trade. That's because serious traders should not use the exchanges' web interfaces - they are not well suited for trading. One should use either self-rolled software (if you can write it) or a professional software trading platform like our Medved Trader - that has superior charting, technical analysis, alerts, scans, DOM, visual trading etc. etc.

We connected it to GDAX so far, will release Binance integration very soon and our goal is to connect it to all exchanges whose APIs are reasonable. So we know these APIs intimately in all their ugliness.

Well, it's a fair exchange judging on the performance. I feel a much faster loading compare with other exchanges, maybe due to the volume of users and trades. (I like the interface too)
If these newbie's target is to trade top coins like ethereum and have no plans in switching to another coin, I don't see any problem in using it.