This largely comes down to personal choice and which look you prefer. Inside the recess is the more popular choice and provides a neater, more compact look, but may not be suitable windows that open inwards. Outside the recess gives you a full view out of the window when the blind is fully raised, it keeps the window sill free to use and minimises light leakage around the sides of the blind, but can appear bulkier from within the room. Both options are relatively easy to install.