However if you are planning on taking profit at $500 or $1000 from your entry then you are better off using the flat fee. However I am pretty sure 99% of people taking bets there are doing it for the short term. So they are all using the former fee option.
It's odd that some people just noticed this particular game in Rollbit when the truth is that it's been around for a while now. I played with it when the fee was zero sometime back and felt it was alright.
The new fee structure makes sense considering how many people were abusing the zero fee feature.