It is a bit of a tough one because it is more down to personal decision in what you like in a car. If you have a budget, you mite want to consider the 350Z as they've depreciated quite a bit more and is probably just as fun to drive if you're looking for a weekend blast around.
Unfortunately with depreciation, it's too unpredictable and can't be helped. But bare in mind that there are fewer 370Z's compared to 350Z's so in that sense it may hold it's value a bit better compared to the 350z's.
Like will said. It's worth test driving one.