I've been plugging away with Pilates for a couple of years. I agree that the side lying series is similar to barre - except that you're lying down!
I only have time to practise barre and Pilates each once a week, and I can't say for sure, but I think that Pilates is helping me to maintain and further the slimming effects from barre - it's as though those deep muscles have been woken up.
I really enjoy the relaxing, focused, precision of Pilates and the stretching aspect, and feeling my body working as a whole.
My absolute fave is Jules Benson's
Total Core Pilates.