Noilly Prat is my preference. It is more or less the same price as Rossi. I like to keep it in the fridge. I feel it keeps it fresher. Fortunately orange bitters are easily available at any better liquor store. I remember having to search like crazy for them 10 years ago.
As for the 2/3 to 1/3 gin to vermouth ratio, feel free to play with it. I like vermouth, but that is a LOT. Personally, I like 3oz gin to 1/2 oz vermouth.
Here is another recipe I just discovered last night, but haven't tried it yet.
1 bar spoon absinthe
4 dashes orange bitters