Speaking from personal experience, I have noticed the Poloniex price swings to be very correlated to how the market is doing overall and how bitcoin is doing.
Bittrex and Cryptopia are as a platform less influenced by market movements, because (probably) they have a lot more investors and longterm holders and experienced people with strong hands.
This has been very good in the past weeks for example. When there was so much uncertainty in the market and Bitcoin could have crashed any day and pull everything down with it. Cryptopia and even smaller exchanges would be defenitely hit the least, as there are almost no people that really panic sell there.
But what about Bitfinex? Well Bitfinex is more tricky actually. Because these very big coins usually have a very strong backbone and their own movements, however, in the last weeks they have all been correlated to bitcoin like no other coin. But most of the big coins are already sitting on massive gains over the last major bullrun and are up around 5-10x across the board actually. Making it even less likely for us to see really big gains on them again.
So this leaves us with Poloniex. The exchange that has all the new people of the scene concentrated at one place. They fomo when the price is going up and they panic when the price is going down. Also, because it is the biggest exchange and the one with the most liquidity they might also have a lot of traders.
So, as bitcoin will most likely go on a tear the next week and push towards new highs, you want to position yourself that way, that all the fomo buyers will buy on top of you. And the place where this can be achieved in the best fashion is simply on Poloniex.