A and B are equivalent under the hood. Your friend is correct that they work the same way. You can usually see that if you look at the execution plan for each of these.

As for which way to write it, I prefer Not In, simply because I find it more readable. That's purely an opinion/preference, with no solid reason behind it. In a simple query like your sample, the readability won't matter. In more complex queries, tracing out all the And and Or operations in a Where clause can be a huge pain, and splitting it up makes that more difficult, not less.

I've seen many queries get wrong results because of incorrect relationships between various And/Or operations, misplaced parentheses, etc. Readability makes a huge difference in how easy they are to debug. Since there's zero mechanical difference between the two, I pick the readable one.