Icardi just seems a far more experienced player who has consistently delivered for the third season running now.
2014/15: 33 games, 22 goals, 6 assists
2015/16: 32 games, 16 goals, 4 assists
2016/17: 19 games, 14 goals, 6 assists
Belotti is the exact same age and has only just come into showing this sort of form. This is also only his second season where he is consistently a first choice player (compared to Icardi who was a first choice striker at Sampdoria when he was 18/19). There's also the ongoing argument about Italians outside of Italy and whatnot... I just think Icardi makes far more sense, whilst Belotti looks like a bit more of a punt. I'm certainly no Serie A expert, I'm just going off of the stats and the limited football I've seen.