1.
Hash tables gives you O(1) access with lesser constant times than red-black-trees, that coud need a few rotations after every insert/remove.
2.
It's better to use cons cells for list construction rather than vectors, because vectors have got an extra cell to store the length of the vector itself. On 32 bit machines a cons cell takes 8 bytes, a vector of 2 elements takes 12 byte i.e. 50% more than the cons cell.
2) In theory, the length of the vector could be stored by the memory manager anyway (possibly as a start and end address), which after all has to know how big to deallocate when the vector is GC'ed. In fact if the metadata is interspersed with the allocated memory it probably will be just a length, since the current address is known.
