These two are quite different in overall presentation, but both have a big sound, both with huge soundstages. What type of sound do you like?
Build wise both are solidly made. The P1 is much bigger though. i struggled to get a proper fit and that was the only reason i returned them. T20 much easier but they can only go over ear, the P1 can sit both ways. the P1 had a detachable cable, real quality one at that.
It was a while since I had the P1 but I've recently got the T20. From memory the P1 is very detailed but VERY smooth and slightly thick, very rich. T20 with the bass filter is a lot of fun.
Bass wise the T20 goes deeper and has a nice sub bass punch, the P1 is overall a bit more neutral and balanced but the bass texture is great. i prefer the bass on the T20 (with bass filter)
MIDS - The T20s are a tiny bit recessed but good overall detail. P1s have great microdetail, they have a really unique signature.
HIGHS - Both got great detail, although this is where I can't really remember the P1.
SOUNDSTAGE/IMAGING - T20 has great depth, P1 has great width. Both present instruments well in the soundstage.
I prefer the T20 tbh. It also has three filters to choose from, and they each sound different. I think the P1 is one you'll either love or hate. T20 is maybe a little safer and consumer friendly.