I think we can say both. In fact, playing and replaying them in my head, I have a hard time deciding what the difference would be. It's not a difference of language register, and it doesn't feel like there is any appreciable difference in meaning either.
Curiously, in the very common:
On arrive bientôt
you cannot use any other word order. Ha! It's possible with the original sentence because the verbal group is long enough! We can also say, on the same pattern:
Nous allons bientôt manger
Nous allons manger bientôt
For other expressions (taken from Luke's answer):
On va arriver bien
doesn't sound very natural at all, because the bien is not very clear. I would have said, e.g. On va arriver frais et dispos, which is now clear. This one is a bit tricky, because on va bien arriver can mean in the end, we'll ge there, the bien being there only for emphasis.
On va demain arriver
is definitely not a possibility. You have to say: On va arriver demain, or on arrive demain, but:
On va doucement lui dire ...
On va lui dire doucement ...
are both possible. So are:
On va vite arriver
On va arriver vite
with maybe a nuance: the first one meaning that we are going to arrive very soon, whereas in the second, we will arrive quickly (an evanescent or mirage nuance: you hear a nuance at first, but when you really try to definitely put your finger on it, it's harder than expected...).
A possible rule could be that the verbal group has to be complex enough to make it possible to insert a word before the main part of that verbal group. But even so, there seems to be quite a few exceptions, so it's not so much a rule as a pre-requisite. If the verb is a simple form such as on arrive, it's not even possible to contemplate putting a word in between the subject and the verb.