SmartRider costs $10 to get the card, plus at least $10 of credit, and in return gets 15% off fares. All money including the card fee is not refundable, so to be better of, you would need at least around $67 worth of travel (at cash ticket prices) over the week assuming you finish with zero balance. This is just over 15 2-zone fares. However there is no discount on the day cap, if you reach that limit than none of that travel will count as a saving - so a 4 hour trip to Mandurah and back from Perth will cost $12.10 with both cash and SmartRider, assuming you don't leave until after 9am on a weekday.
Using autoload will get a further discount (but is probably too much hassle for a short stay).
One other benefit of SmartRider is free train travel in the CBD (between City West and Claisebrook, and Perth Underground to Esplanade.) Using cash tickets requires this to be paid for as with any other journey, though of course this is included on any ticket valid for zone 1 within the 2 or 3 hour time limit. CBD buses are free to use regardless though, and that is not just limited to the "cat" buses.
(anuddernut, DayRider works anytime after 9 on weekdays - it's the concession free travel that ends at 3:30.) http://www.transperth.wa.gov.au/Tickets-Fares/Special-Fares