I think either one works pretty well, but I prefer a stationary bike! To me, it's more fun than a treadmill. I watch the news without sound and listen to upbeat music and end up doing 45 minutes at target heart rate around four times per week.
It's really firmed up my legs well without making them bulky. Use arm weights to firm my arms, and with low fat eating I have managed to stay the same weight I was in high school.
I used to work out with weights, but they do tend to add bulk and make you weigh more, although I did lose inches. The weight lifting made me super hungry though, whereas it seems as if aerobics suppress my appetite. Also, I found that 40 plus Xenedrine works well in aiding fat loss and doesn't make you nervous.