If Prussia is given Poland, Rhineland would be given to other state, likely Hannover, otherwise balance of power would be disrupted. Prussia would look east ITTL, initially hoping, that Poles could be easily Germanized, time would prove how wrong they are. Over time Poles would dominate Prussia from inside! Prussians would certainly manage Poland better than Russians and Austrians, they managed their part better IOTL, Prussian Poland would not be cut off from the Baltic Sea, serfdom abolition would happen earlier and would be done better, Posen and West Prussia IOTL were far richer than Russian and Austrian Poland by the turn of 20th century-almost 100% literate, with bigger farms and agriculture as productive as in Western Europe, more railroads, regulated rivers (so Vistula below Warsaw would not be "Polish Amazon" anymore

) and so on. Differences are still visible even today.