I like windowsills personally. Assuming those are my only two choices. It allows the plants to grow to their full size, where the terrarium may not be large enough. Also, sometimes terrariums may have a problem with stagnant air, i'm not sure about this, but it seems to me like it would be a problem. Besides, most highland Neps, sarracenia, flytraps, many butterworts, bladderwors, aldrovanda, many drosera and more all do well on windowsills. I have highland neps, sarracenia, dionaea, and a butterwort doing very well on a windowsill. My friend Andrew has a few bladderworts, butterworts, and a N. Morganiana on his windowsill. Morganiana is a lowlander by the way. So like windowsills best since most cp's can grow there anyway, they have better air circulation, and they provide plenty of room.