Wow there is a lot for you to think on and I can see all sides to the discussion.
On one side if you like the treadmill then that means you will use it and will be more likely to stick with it.
On the other hand I agree with Em and Tom, walking/running is free and I must admit I prefer to do both outdoors but still do use the treadmill at the gym. So getting the rower or cross trainer would certainly work you in a different way and give you variety to your workout.
Personally if I had the cash and space I would get the cross trainer as I regularly walk anyway and the weather was never a stumbling block for me as I began my love of exercise in the winter! Plus I just love the workout the cross trainer gives me.
Maybe try this. Go to a gym and get a free day pass then try out all their cardio equipment and see which you prefer. It sounds like you have used a treadmill and if you bought something you're not used to you might get put off it if you don't master it first time out. Just don't tell the gym what you are doing!