Rowing machine or treadmill for weight loss and slimming down

I (38m) am looking to get a piece of exercise equipment for the winter and was hoping to find something that will burn calories and tone muscles (and help my butt if possible) without bulking up my shoulders - I am hoping they will slim down as I lift less and do cardio.

I have really wanted a Concept 2 rower but am worried about making my shoulders and back even larger. I have also been looking and the Versaclimber climbing machine or a treadmill. Hoping to start HRT by end of November.

Other than body weight exercises and things like mountain climbers and Pilates what would be the best equipment to get to do cardio and slim down indoors without a gym.