$600 for a two month old V3i seems awfully cheap in my mind. I'd ask for the serial number and then call Whites to verify that the machine is legit. If it is, I'd jump all over that deal. The warranty will carry over (a 2 year warranty) so you've got plenty of time to get it fixed if there is something wrong with it. Whites customer service is legendary.
I know absolutely nothing about the AT Max.
In my mind the two machines aren't remotely in the same category. I'm sure others will disagree, but the V3i is a much more sophisticated machine with far more capability. That can be a great thing, or a terrible thing; depending upon your point of view.
The V3i has the best screen available. Nothing else comes close - unless one of the new (yet to be released) machines challenges it. The V3i can be adjusted for any condition - to your liking. You can change the frequencies, the tones, the threshold, the discrimination, the colors, the.... etc... Sometimes those changes end up making the machine less capable.
You can also hunt in one of the base programs, with no tweaks. You'll find stuff in those programs, but it won't be the ideal settings for the conditions. Tweaking those settings can make the machine go deeper, faster, etc.. Or it can make the machine less deep, slower, etc... Start with the base programs; once you've learned how they work then make small tweaks to the programs. It isn't all that difficult to learn the V3i; it will take some time and experience with the machine.
If you do change a setting that makes the machine less capable - it is easy to go back to the stock program.
Assuming the serial number proves the machine to be a legitimate machine, I'd watch all the V3i videos that Whites put out. They'll give you a good idea of how the machine operates, and how to change the settings.
Someone else will have to tell you about the AT Max. I've never used one.
