Well, I had a big post here but the forums ate it, so I'll give a TL;DR of it.
Giroux has a better instinctual base compared to Pageau, in terms of reacting properly and creatively in stressful situations and when surprised/pressured. Pageau can read the game very very well, and plays it very very well, but he can be a bit straightforward when put in those high pressure situations and I'm a bit concerned that his ability to make his teammates better won't translate at the NHL level. Giroux for certain does make his linemates better. Pageau, I'm not convinced, and I think he'll be easier for players of NHL caliber to read.
However, he's tireless, he's so committed to improving, and he thinks the game incredibly well. His size hasn't been a real factor, and with his ethic, I'm not sure it will be a huge obstacle.
Don't listen to the comparisons to Giroux, he certainly has that fire, and plays a similar game for sure, but he lacks that upside IMO. That said, I think if things pan out he could turn out like a smaller Patrick Sharp.
Essentially, there's a small difference between the best scorers in the world and guys who put up 65 points. What few things Giroux does better than Pageau are exactly the things that made him elite from the get go. Could Pageau miraculously obtain such abilities? I guess it's possible, but I've rarely seen that kind of talent improved upon at such an age.
My advice is, if you're in a deep enough league, grab him. If he lights the playoffs on fire this year, use the ensuing hype and trade him. Unless you're in a dynasty league where depth is everything, where it would be best to keep him unless an incredible offer was tendered.