I'm not on poloniex so it's hard to give an opinion about them. I'm on Bittrex and Kraken, bittrex is a great platform but is kind of a trap because there are so many altcoin, you can do easy mistakes if your not an experienced daytrader.
On kraken, there are less coin and almost all of them are solid project, it may be harder to lose in the long run.
Finally, i don't think we can trust chart in this very moment, we are in an uncharted territory and in the short term, i don't want to be optimistic and misguide people. The fork could cause a serious shitstorm if things go bad and a giant pump if all goes right.
Everyone who want to invest in July have to choose a belief to make a decision.