The 2 engines are chalk and cheese.

The P100 is almost P70 size and the specs show it being slightly more powerful than the P80. I haven't confirmed the weights but have no reason to doubt them. Jetcat usually post the weight of the 'core' engine and you have to add the weight of the ancilliaries to get to the installed package weight. However, the P100 has the valves (and Kerostart) in the engine case so the published weight for the P100 already includes both solenoid valves. It also comes with the V10 ECU which is smaller and lighter than the ECU that comes with the P80. Overall, the difference in the installed weight between the 2 engines is probably even greater than you think, with the P100 being MUCH lighter.

The only real reasons I see to buy a P80 is that it is cheap, very quiet and proven over a huge number of flying hours worldwide.