From the 5th, I'd think a cab would run between 50E and 65E depending on route and traffic. I'd still do that rather than get a cab to the bus-too much hassle and the big "if" is getting the cab in the first place.

As for AF vs. Roissy, use AF. They leave like clockwork and are very reliable. The Roissybuses have an awkward system where luggage is taken on the bus, and often passengers leave bags in the aisle, forcing others to step over them. AF has handlers who stow bags under the bus and others who take them off when you arrive.

Having said all that, unless you really have a ton of luggage, walking from your hotel to the St. Michel RER station would take about 10 or so minutes, and it's a straight shot from there on the RER B. Just keep an eye on your luggage.