I think the other thing to bear in mind is the reason people awant to purchase either car. Certainly the application it is to be used for by the purchaser is also going to have some level of influence over the future price of either the 350 or the 370.
For me personally there's a more emotional attachment to the 350 than the 370, but that's entirely down to getting to drive one on a track all those years ago and thinking "I need one of those!" Its also older so therefore requires more jusitifiable maintenance and upgrades due to age of the OEM components. Yay modding time!
I love the 370 as well, it's a much better car in many ways but by that same token that means there's less to change or improve IMHO. Vastly better interior for example and rev matching being two great assets for the 370, but then again would you prefer a full retrim and heel and toe in a 350? Its down to the individual choice and neither is right or wrong and that's what makes these cars fantastic.
Over time as we go out to the far future I suspect there won't be much in it pricewise. Both will be classic big engine RWD sportscars, they are just on different points of their curves and you see that with virtually everything. Cars, bikes, furniture, computers, you name it they all start relatively at a high, drop low and when there's virtually none of them left become expensive again that's just the way the world works.