The 4200 is brighter, clearer due to it's enhanced coatings and is also stronger in construction. However the 3200 is no slouch and as James mentioned is quite possibly the best scope for the $. Is it worth the difference to buy the 4200 over the 3200, well only you can decide. However if it is your primary deer rifle that will see lots of use in all sorts of situations (and you have the extra money) I'd opt for the 4200 personally. I own both and picked the model based on the usage, the 4200 is topping my ML mainly b/c I use it 2nd most of all my guns for hunting & the 3200 sits on top a another rifle I rarely use for hunting anymore(this is not a slam to the 3200 just I have many guns and they all need glass). I would choose either of these over a fixed if you'll have both short and long range shots with the 7mm-08, if your thinking mostly close shoots the fixed would be a ok choice.

Best thing is do some looking through the scopes of interest both in dayligh and low light situations, then pick the scope that is best for your funds and application.