When it comes down to 1st or 2nd focal plane, the big dealio is whether or not you're going to be doing a lot of target ranging quickly and on the fly. If you're not, then don't get the FFP. The FFP allows you to range unknown distance targets at any power range, easily.
The drawback to the FFP is that when the power goes up, the reticle gets larger and coarser. When the power goes down, the reticle gets smaller and finer. This is pretty much ass-backwards to how you want your reticle to be in most scenarios when changing power.
The SFP reticle stays the same at all powers, but is set to range at a single, pre-set power. Although, any of my students can tell you, you can easily range with an SFP at any given power, if you got the know-how. This kinda makes the single "advantage" of the FFP a non-issue.
Personally, I only use SFP scopes.
Usually, diopter adjustment means that you have a parallax adjustment that is tuned by turning the outside of the objective end of the scope, i.e., what is usually referred to as an Adjustable Objective, but it can be used differently than that. Can you cut and paste the exact reference to diopter adjustment you are inquiring about?