I'm very familiar with both Motorola and Harris P25 equipment.
If the two system brands were identical in functionality and performance, the dramatically superior design and quality of Motorola's subscriber equipment (primarily the portable radios, which represent the bulk of the money for subscriber equipment) definitely gives Motorola a huge edge. Put simply, nothing Harris (or the legacy brands before it) can hold a candle to the venerable XTS5000, and the newer APX radios are superior to the XTS line by a respectable margin.
Harris is way behind the curve when it comes to the design of a portable radio product for public safety use. While the M7x00 mobile radios are very good, and quite competive with Motorola's latest mobile radios in quality, Motorola dominates the portable radio side of the market and that is arguably the most visible part of the system, and the one that the greatest demands are made on.