Creation: model based levels are more difficult to texture than blocks.

Performance: BSP tree culling is able to cull away most of the geometry that is not visible, this is very good for performance. It works best in indoor levels. However, it cannot take models into account, so you lose this advantage. If you use the upcoming ABT, you can import entire levels from external modellers (with arbitrary geometry) and it will perform efficient culling (although not as efficient as BSP).