I am going to shy away from specific recommendations but compare the two models and let you decide what makes more sense in your case.
Unibody mac book pro are made of aluminum which is more prone to scratches, gouges and pressure wearing if grit or sand gets between a rigid case and the mac when compared to older MacBook Pro and similar era plastic MacBooks. (but not nearly as soft as the current shiny when new unibody MacBook construction) This coupled with the great increase in strength due to the unibody makes vinyl stickers a more practical case than with the older macs. (you don't need so much hard shell protection and a thin layer of cosmetic protection might be all the protection you need to invest in)
The new macs are more prone to break rather than bend like the old macs would and they take a whole lot more abuse before showing it in practice. I have seen many portables of both era that basically flew out of a speeding car and landed suddenly on pavement. (Don't forget your mac on the roof of your vehicles, people, even if it's in a nice case.)
Since the 15" mac is larger and heavier, so more padding in a bag or case is needed to protect from drops. Especially drops from waist or chest high to hard ground. It is also more expensive to repair your new mac than a smaller portable. The repair costs for 15 and 17 inch models are capped at $1250 (US prices from Apple for substantial internal and external damage). The 13 inch and smaller macs repair top out just under $800 for equivalent damage. You might consider investing more in a protective case or bag based on the potential for increased repair costs.
Don't forget that even in a cushy padded bag, high G forces or bending can still break internal parts or the glass screen/trackpad even if the external case looks barely marred. The forces needed to leave a half millimeter bend in the case is enough to snap internal parts due to precision design packing maximum function in a minimum space.