Poloniex is a very good exchange especially for STEEM, they are not so strict in aspects like booting you off when you didn't use the site for sometime because of their terms of use.

With regards to these products you are discussing here, it is a good option to park some steem and earn a bit from their APY offer if you plan to sell it but not immediately. I don't know why the APY for larger amount of investment will have smaller APY though.