Btw. this is somewhat ontopic: because there were many scams, hacks etc. in crypto world it's probably worthwhile to check whether there are any red flags regarding particular exchange. Especially regarding regulation, audits, unbusinesslike conduct etc. A good site for that is http://digiconomist.net/fraud-risk-assessments - although in some cases an important piece of information might be missing and in other cases the risks might be overblown. For me personally the most trustworthy exchange at the moment is Bitstamp but unfortunately there've only three cryptocurrencies, no margin trading and high trading fees. Kraken, Bitfinex and Poloniex are certainly interesting exchanges for automatic trading because they provide many trading pairs and zero trading fees for market makers but they are also less reputable. I think the way to go is to split the investment among several exchanges.