I just had the exact same question. I have a pair of Burton Motos that fit me perfectely and I saw a pair of Burton Ions online that were really cheap so I emailed Burton to ask if they would fit the same. They said that they would fit the same, but they would feel different due to the difference in stiffness and flex.
Anyways I ordered the Ions and they fit and feel great. I am not sure if the Ambushs will have the same sizing, but my guess is that they will. If you're still unsure email Burton. They gave me a bit of a wishy washy answer when I asked, but I think that they didn't want me getting angry at them if I didn't like the new boots.
Here is a copy of my question and the answer I received:
I have pair of older 2008ish Burton Motos that have fit me perfectly since I bought them. I was thinking about upgrading to some Burton Ions and I wanted to purchase them online, but I'm not sure how they will fit. Will the current Burton Ions fit my feet the same as my older Moto boots?
While the sizing of the motos and ions will be the same, the cut of the boots may cause them to feel differently on your feet. The Ions are also going to feel stiffer than your motos so that will give you a different feel as well. I would highly recommend trying them on first if you can so feel them on your feet. If you have any other questions please feel free to ask.
I'm am trying to get a stiffer more supportive boot than the Motos because I just bought stiffer bindings and boots. I understand that the Ions will feel different then the Motos, but as long as they are basically the same width and length I think i'm going to buy the Ions online because I found a deal on them for $200 that I can't pass up.
Thanks for the response!-Ben
Overall, they should fit very similarly. If you can find a deal then that is great! The Ion is a great boot. If you have any questions please feel free to ask.