Hi,  The difference is simply that one is installed in the wall and the other is self-contained for placement in the room on the floor.  There is no requirement to install both.  Most in-wall subs are selected for aesthetic reasons to conceal the sub.  The quality of bass is a function of the quality of the in-wall sub itself although some integrators discuss issues with wall rattles and possible wall cracking.  These kinds of issues are often to be blamed on the installation and not necessarily the in-wall sub itself.  There is no inherent reason a floor standing sub is superior, although being able to make placement adjustments to smooth the sound is a very useful advantage.

Most choose one type or the other.  Some might mix an in-wall with a floor-standing sub but my recommendation is to use identical subs.  This is true no matter how many subs you buy.