I like the idea of placing a sub under the seats. When I was younger I had twin subs in the boot, but now I need that boot space for pram etc.
As to which to suggest it depends on your car and you music tastes, but mostly your budget.
RMS and frequency response are the key indicators of quality. Ignore PMPO - it's just a vague figure that some manufacturers use. Fly make IMO very good quality ICE components.