What made me decide was the power rating. The L5 can handle more RMS power with a smaller foot print. I believe the L5 also has a bigger and heavier magnet. In my opinion the pioneer was made to fill a nitch in the market but subs designed this way will not sound as good as the traditional subs. Otherwise there will be no reason for manufacturers to buld these giant subs.
I have not heard the shallow pioneer and I can be totally wrong but it seems like the better subs are heavy and deeper.

The deeper the sub the more room it has to move up and down in turn moving more air.