I wouldn't dispute the Rashford hype sentiment (a propaganda wankfest accompanies every rag youth academy graduation as a matter of course - and the fact that Rashford was only given a chance because there was almost literally no-one else fit is naturally ignored by the media), but I've not seen much to support your assertion that Kelechi is "clearly better than him" and the claim that he "would have performed better than him if given the chance" is pure speculation.
Kelechi has scored just twice in the league (no-one could seriously count the back deflection vs Swansea), and one of those was a tap in from two yards at Palace.
I like Iheanacho; he's an excellent goal poacher in the 6 yard box and a smart finisher generally, but outside the box I would say he's been good rather than eye catching.
Much as it pains me to say it, Rashford looks a menace every time I see him. He's tall, he's lightning fast, and he's not just scoring goals, but scoring them in big games (Arsenal, City, West Ham). Kelechi might go on to become a top, top striker, but if I'm being cold blooded on what I've seen so far, I'd take Rashford if offered a choice between the two, although the call is undeniably a close one