i'm a rower and a bit of a runner for triathlon purposes. i have a concept 2 rower and is deemed an industry standard as it is a bloody good machine. but in your case i have to say if your overall goal is to run a marathon then a treadmill will be the thing for you, you want to be fit for running not rowing.
everytime you run (or any activity) you are acting to get your muscles fitter in THAT sport, i.e increasing muscle efficency, and if you are planning to do a marathon the efficency is going to be very important.
rowing does use about a huge amount of the muscles in the body, and mainly the large muscles (quads, arse and shoulders), thus providing a good overall work out for the heart as rowing produces a good load on the body. however rowing is not a load bearing sport and as such your heart rate isn't as high as when you run. in general i burn more energy per hour in running than i do when i row, though this maybe because i am fit for rowing and not running. the only problem maybe that you are running to lose 2 stone, you say you have run before, would be that would your body be able to take the strain of running twice a day?
but anyway i would go for the treadmill over the rower
WN