I think Thielen is WAY more physical, he simply isn't going to be denied when the football is even in his vicinity.
Diggs just doesn't seem interested in contact while Thielen welcomes it.
I've seen Diggs not try often if a ball doesn't drop right into his hands while Thielen would be the dude who would dive full speed just for a chance at a catch anytime.
Doesn't make many contested catches... Thielen is just selfless while Diggs seems to be going Diva Selfish route which is a shame.