X-Terra 50 at this time. To be quite honest, it might even be the X-Terra 30, and here's why I feel either of these two might be the way to to.
There is no "best" or "perfect" detector out there and it's to anyone's advantage to have at least two detectors that will compliment each other. Therefore, you if you feel you know the Explorer II well enough and are comfortable with its performance, then keep it. It's presently your "main detector" and it might well remain so.
Then, if you know it well enough, consider its weaknesses or those times when you would like to have something else for certain applications. Choose a detector that will handle those needs better than the Explorer II. Pretty simple.
One of the detectors in my personal battery that I totally enjoy using is my Shadow X5. I enjoy using it because it has ample manual control to satisfy me and get the job done, I have a versatile 3-coil selection for it, and it is very lightweight and comfortable for me to use for long periods. For me, it is my "main detector."
But there are many people who would not be comfortable with it because it lacks visual Target ID. I do not
need TID, but often there are times when I
want good, functional TID.
You might be one of those who wouldn't want to be without some form of visual TID and the X-Terra 30 & X-Terra 50 provide that. In addition to having an easy-to-view display they have easy to adjust controls. The only factor that might influence your decision right now would be the fact that optional coils are not yet available and you might want to know what they will be (size & type).
The X-Terra 50 does provide some extra manual control, a little more refined discrimination selection and an extra Tone ID. The main thing the X-Terra 50 will offer is the ability to opt for a coil for operation at either the 'standard' 7.5 kHz or the higher 18.75 kHz. The main thing the X-Terra 30 offers is "turn-on-and-go" simplicity.
One thing they both provide the end user is a comfortable, lightweight design and very good field performance. I would encourage you to contact a Minelab Dealer to get a hands-on look at these two, side-by-side, and make the decision that way. You might end up using your Explorer II a little less and assign it more specific duties rather than be a general-use unit.
Best of Hunting for that new 'back-up' or 'primary' detector!
Monte