Generally speaking, a newer version would probably be preferable to an older version, because it's expected that the manufacturer is fixing any issues and introducing improvements and (hopefully) additional features from one version to the next. Unfortunately, that isn't always the case, because new issues might accidentally be introduced, or the manufacturer might switch to using cheaper parts in order to reduce manufacturing costs, or less sturdy parts in order to reduce the product's weight.
I'm not familiar with the specific differences between Axiom v1 and Axiom v2 models, but I have an Axiom 61-II (v2 or 2nd generation) and am happy with it. Unless there were significant fixes and improvements from v1 to v2, the v1 should be a good choice-- although one thing to keep in mind is that both the v1 and v2 are old, so both will be used, in which case it could be tricky knowing which one has been "used and abused" the longest, and which one's owner was the most diligent about taking good care of it.
Aside from age and features, another consideration is the number of keys. Normally you'd want to get the greatest number of keys that you can afford-- 88 being the ideal number-- but this also depends on how much space you have for setting up the keyboard, and how portable you want it to be. I haven't done an item-by-item spec comparison, but I think the Axiom and the Impulse have comparable features and capabilities as far as pads, sliders, buttons, knobs, wheels, aftertouch, etc.
So my own preference would probably be to get the Novation Impulse 61 if space and portability isn't an issue, simply because it has 61 keys. But I'd probably get the Axiom 49 v1 if I were looking for something a little smaller, simply because it has a lower price. But either way, I'd want to be able to check out each one before deciding, to see what kind of condition it's in.
