Any advice would be much appreciated!
Ultimately you'll decide by your interests, budget, space on cookbook shelf.
But here are a few thoughts to put in the mix:
1) It's better to buy either a hardbound (if you like books) or kindle version...I got a penguin copy of volume one when I found it in England before it was out in the USA and basically it was soon living on life support (rubber bands, pages falling out etc....no bad on Penguin's part, just heavy use). Strongly recommend you get a format that you like & is durable/flexible enough to add your notes and results, preferences, changes.
2) Another thing - if you didn't know it - Child and her collaborators Beck & Bertholle (Sp?) took about 10 years to write in English a humongous manual of all French cooking and couldn't get it published. Only when the masterful Judith Jones made a limited selection did the book we now know as volume 1 emerge.
So, partly, volume 2 reflects both the original work of the 3 collaborators and Child's later editing after book 1.
3) Last, look at the contents and/or index of volume 2 before you buy...on Amazon, in a bookstore. It has more of good things you'll make all year, but also specialty knowledge neither the English speaking nor French home cook needs or wants (baking baguettes, butchering etc).
Whatever you decide, have fun cooking your way through the book(s)!
I suggest you just start with Volume 1, it's a handful by itself. Volume 2 adds considerable coverage on baking and charcuterie, so if those two topics interest you, you should consider eventually picking up the second volume.
I bought my copy of the first book years ago, twelvth printing from 1966 (four years before Volume 2 was published, so there's no volume number).
Budget shouldn't be an issue. You should be able to find used copies from $5-15 (shipping included) at a used bookstore or Abebooks.com.
I should note that I purchased my copy used, so I definitely did not shell out much money for it. I'm glad the volume sits on my bookshelf, but I am also glad that I didn't pay much for it.
Anyhow, best wishes for your purchase decision.
It seems to me that they are often sold as a set, so maybe that could be your deciding factor.
Otherwise, the Foreword to Volume II does a great job of describing the additions. Besides the bread and charcuterie, there are "informal vegetable soups," more fish stews, variations on poultry and meats, including a focus on less expensive cuts (at least they were at the time), and some original (opposed to classic) vegetables.
When I went away to college, my mom bought me both and a lot of wonderful kitchen equipment. It took me a long time to work my way through volume 1, so no rush, but I'm happy that I have both.
Depends upon what you're planning to do with them. If you're planning to cook your way through them, you might prefer one at a time. On the other hand, both are excellent reference books, as well as cookbooks. Buying them both at the same time will give you a trove of information