Hi2050 and pro80 are really similar.
-Pro's have more bass. Closed headphones usually do.
-Pro's have bit more highs, but highs on hi2050 have more air and sparkle. It's because of the velour pads and propably the semi open structure. Mostly velours though, it is that very distinct velourpadhighs sound
-Hi2050 have slightly smoother warmer sound. Fabric covering the driver does this. Pro doesn't have this fabric so that is propably the reason why the highs are more pronounced on pro. Some people will propably say that pro has smoother sound. On bassy tracks that might seem so but when listening to mid centric music it is obvious to my ears that hi2050 is warmer and slightly colored. I tried pro's with the fabric and with that pro's mids are about identical to 2050.
-Hi2050 leaks sound, Pros isolate very well.
-For some people hi2050 is more comfortable because of the velour ear pads and because they feel more airy on the head due to the semiopen structure. For me there is no difference
-Pro80 has better headband padding and fancier plug. Only cosmetic improvements.
-Some pro's have coiled cable
-Pro's come with nice carry case, 2050 has extension cable
Those are so similar that I'd just buy the cheaper one. Which usually is hi2050. Pro is technically more impressive because it is closed and still sounds equally good to hi2050. There propably are some differences in soundstage and imaging. I haven't compared these side by side that much yet.