Looking at the Hop on Hop off website, it looks like it is the same thing (somewhere on the website it says it is run by the Big Bus). The cost of the Hop on Hop off bus tour in Paris is only €32 (for access to two lines) or €33 (for access to four lines). Your round trip RER ticket would be a bit more than €15. I looked, and one of the Hop on Hop off lines (the blue line on their map) has a stop near Gare de Lyon (which is the easiest place to get off the RER A line without having to transfer to a metro line). So you could still do the bus tour, but not have to worry if at some point in the afternoon you decide to sit at an outdoor cafe and watch the world go by - because you wouldn't need to rely on the Big Bus tour to get you back to DLP.