keep in mind that when your solar panel voltage is for example 6V, and your circuit will decide it's high enough to switch your load to it, then your solar voltage will drop (because of the current you draw from it) , your circuit will decide the voltage is too low, and it will swithc your load back on the battery. Because the load is not on the solar panel anymore, the solar voltage will rise to 6V again, and your circuit will decide it's high enough again.
etc.
this will cause the the system to swithc between the two sources again and again... not good.