Mario and Kristaps were playing basketball from very early age and moved to Spain at the age of 15 to attend basketball academies, where they concentrated on improving their skills. As far as skill development goes both are very far on their developmental curve. They naturally become better players as their bodies mature, but I wouldn't expect massive expansion of their skillsets.
College teams OTOH actually do not concentrate on player development, so guys, who show significant improvements year to year, likely have excellent work ethic, since they have to work on their own a lot.
So here's an outrageous thought: WCS at 22 y.o. has more untapped potential skillwise than both euros at 20. For the same reason I don't think Hezonja is going to be a much better player than Ben in 5 years. Mario's biggest advantage over Ben would be his size/length.
Porzingis is maturing/getting stronger and you can see it in many things: his FT rate almost doubled from last season, and went from atrocious to just below average as he's able to be more physical; he fouls less as he deals with physicality better; defensive rebounding rate risen from bad to average as he holds position better. At the same his percentages from the field and passing effectiveness stayed the same. So when I posted those block/PF numbers I implied, that Porzingis is a good shot-blocker, but is pretty far from elite ones coming out of Europe recently. As for physicality and body type he's nowhere near ready to deal with strength and quickness of NBA players, and when he bulks up, Kristaps might lose a step like Bargnani, Valanciunas and Motiejunas did.
P.S. Like my highlight mix of WCS better
: out of all dunks I left only those, where he has to take steps or a dribble after catching, or where he makes a play near his team's basket, and then is the first to arrive on the other end.