I wouldn't be shocked with either making it to very top. But I guess we should have learned by now that it is very very hard to predict those things. Given that both are very young (by the looks I could easily five FAA 16 or even less), obviously they can go up and down a lot. So I guess that I am bound to see they get in a rough patch for a month or so and people come and say that they will "never" be nothing...
As I posted in the tournament thread, yesterday I watch a (almost) full match of FAA for the first time. I was positively impressed. He seems to have a fluid, beautiful all around game. He got out of jail with his serve a lot of times. But it was a match full of errors from both sides (which is understandable). Shapovalov right now is more mature, and seems to have a higher ceiling. But, if you take possible injuries into account (as the OP does), than I can see the sense in the prediction. One thing I remember is that Federer has chosen FAA to be his hitting partner in this pre-season. I guess he knows what his doing.