Ivy,
From personal experience and what I have read, if you want to build serious muscle, you need to use weights. I dont mean like a body builder. When I lift regularly, my body looks "tight" and is "firmer" to touch.
As we age we lose muscle mass. Lifting weights is the most effective and accessible (based on limitations in range of motion of the average person) way to counter this.
Pilates, I dont do, but I do yoga. The really strength building forms of yoga like ashtanga which can help you build muscle using your own body weight as resisitance require near-gymnastic skills. Yoga works synergistically on flexibility, strength, endurance and balance. So if your body structure is not conducive for one of these, it is difficult to truly get the best of the muscle building benefits. Yoga can make you very strong, especially in your core. But only the super-proficient (with balance and range of motion) are able to build muscle mass from yoga. From what I know of pilates, it has many similarities with physical yoga.
I personally think lifting heavy-ish even if just once a week and mixing it with pilates / yoga the rest of the time is better than just doing pilates/yoga. I also think lifting without working on forms of exercise that focus on flexibility is not a good idea. As we age apart from muscle mass, we tend to lose range of motion too, unless we work on flexibility. Range of motion is as necessary as muscle for a body to truly look youthful and is important for functional reasons and to prevent injury too.
Another good thing about doing yoga/pilates is that even if gymnastic contortions are beyond one's body structure / aptitude, one gets relatively flexible and improves on balance with practise.
ETA: I like what Clare said. I dont believe you can lengthen muscles. When you do exercises like pilates your muscles respond in a different way (not as much mass) as when you train with weights. So the "look" of someone who trains with pilates is diffferent from someone who lifts heavy on a regular basis. Genetics has a lot to do with it too, because not all people who train with weights develop big muscles. Similalry, if your natural body build is stocky doing a lot of pilates wont turn the body sylph-like.