It depends on if you see Holland maintaining his position in the pecking order come rwc. I disagree with Dave that rwc is an afterthought. I think it's front and center for Schmidt. He has spoken fairly strongly about aiming to be 3 players deep in every position by 2019. Holland isn't the future long term, as you say Hugo, but he will only be 33 and if we have injuries he may be who we end up looking to.
I really don't think Toner should go. He might be in NZ, but even if not, we don't need to know anything more about him. If Henderson isn't with the lions, then I would possibly take him, but (and not everyone might agree) I think he is also nailed on for 2019 if fit. After that we have a problem and unless we grow someone new by 2019, then Schmidt may have to consider bending the rules for D Ryan. That's an option, but not ideal. Would've been a great trip for Dillane, but it's not to be.
On the basis of all that I would definitely take Roux. He really impresses me and whilst I haven't seen every Connacht game, he has looked good to me in what I have seen. Treadwell is worth a punt probably too. Big and fast, but he is the opposite of Holland in terms of smarts, at this stage of his career. He has a lot to learn, I'm not sure he'd have overly impressed Schmidt just yet, though some of his highlights are definitely eye-catching. Molony is also in the mix for me, given that he has shown potential to improve, but it may be too early for him too. To be honest I'd go Roux, Treadwell and it's a gut feel between Holland and Molony as to whether you feel in 2 years time Molony will be worth that investment now. I'm veering towards feeling he is. But taking the 4 of them might be the fairest thing of all. Let the players decide it themselves out there and that would potentially more help grow the depth we need in this position.