Who do you suspect will be the better player in 3 years?
It's hard for me not to be biased, but I can't help but think Fox has a good shot at taking the crown in the next couple years. If he shoots even a couple percentage points better from 3 and the line you are looking at a monster scorer. He has the tools to be one of the best defensive guards too he just hasn't shown it. Maybe Off Night rubs off on him and he elevates his game on that end.
Yeah it's tough. It's hard to imagine Fox getting much better on the offensive end than he's been the last few years (improved shot willing of course) and that's about all you can expect out of your #1 option. He's got to show the growth on the defensive end, something usually pretty rare for a player heading into his 5th year.
I think short-term, I'd still take Simmons the next 2 years. If you told me I could only have 1 over the next 5 years, I'd probably gamble that Fox hits his defensive upside more often than not.