Holly is certainly correct in terms of trying a range of scopes. Bear in mind that there will be variation between scopes of the same make and model, so if possible, try the specific scope you intend to buy.
Big Nikkos are very popular, and for good reason. A decent one will range find very well and perform well in most conditions. No massive shifts due to temp, but can white out depending on sun etc. They are heavier than some other options though, but a bargain compared to many.
I have 2 Big BSA's, the fore runner of the Big Nikko, which I really like, and a Mk2 Big Nikko which is also very good.
As for the Mk3 being a step up, not convinced about that in reality.
It will come down to how much you want to spend on a scope. You will get a Big Nikko/BSA for not too much over £300 that will repeatably range to within 2 yards anywhere out to 55 no bother.
Up from there will come down to what suits your eyes and your budget