I guess there isn't anything wrong with waiting for it to come on Steam, but even though you have to buy it over their website, a Steam code is sent to you in an email. You just redeem it like any other code and you're ready to play. The game will go in you Steam library like any other game on the store.
And, you don't need an account for PayPal (unless it has changed since the last time I used it), you can just pay as a guest. I think it is similar for Amazon, not quite sure though.
Also, the game will update the same way whether you buy it now or over the Steam store.