Regarding which cabin to choose...higher are usually more expensive but not always better! the less bumpy ride, should it be choppy, is in the middle and as low as you can go!! We usually stick to the middle and try and choose a deck with cabins above and below you rather than the theatre, bars, etc. If you go too high you may have the crew positioning the sunbeds at silly oclock in the morning!! Have a look at the deck plans and you will see which are the bigger cabins/balconies and what is above and below! We did an itinerary last summer on Sea Princess which called at Monaco so perhaps consider a Princess cruise. Happy cruising!
What ship/ cruiseline are you looking at? Cannes to Monte Carlo is a simple train ride. check this site for timings: http://www.voyages-sncf.com/dynamic/_SvHomePage?_DLG=SvHomePage&_CMD=cmdHomepageUK&WB=HP.
General rule of thumb the higher the deck the more it costs. I would agree a balcony is a great assest.