It doesn't make too much of a difference but from the aspect of adjusting to different cultures, starting in North America allows for an adjustment to travel by starting in an English speaking environment. You can practice a bit on your spanish ahead of time and the more you pickup prior to full immersion, the easier that transition will be.

The other thing to keep in mind is that access to bikes and gear is very good in the US, has to be one of the cheapest places in the world to buy a bike and gear. Once you get into South America, the market is a lot smaller and there are significant duties in most countries so you will get a lot more bike for your dollar in the US. The only exception to this general rule is if you can find a traveller bike to buy off a fellow traveller in South America.

And due to the cost advantages mentioned above, buying a bike in mainland USA will save you a fair bit on shipping. So if you have a bike in Oz that you know and are comfortable with and it is available on this side of the pond, I'd look at lining one up ahead of time and then landing and heading off. As you ride, you will tweak your gear and it's dead easy to order things online here for cheap and have them shipped and waiting for you at a town down the road. As you move south, acessibility and cost become issues to doing this.

Happy planning.