G'day Vince,
If you have been shooting the 36x then I would recommend you go for the 40x.
I have shot a borrowed 50x March, and quite frankly it was good, but the mirage on the day defeated me. In clear conditions the 50x is great, but we have a lot of mirage in Australia, and in South Africa it was fierce at times.
Perhaps the answer for you might be the March 36-55x variable. I have shot some groups using one of these, and just maybe its the best of both World's where mirage may be a problem.
Brendan