I would think slim series. B/C it is geared to max your wgt loss and p90x is build body muscle from what I read on the info book that came with the videos I have. I have the 90 boot camp videos I can not think of the name but it is with Tony. It seems pretty cool but I really like Cathes work out and style for me better. It is easier to build muscle when you are eating a regular balanced carb/protien/fat. Then if you are doing low cal /low fat you need the balance to build muscle. Hope it helps.
I "heard" the Slim Series was super boring. P90X does build muscle mass but that will help you burn fat...the more muscle mass you have, the better your body is at burning fat!