Right now you have a very good detector. One that has provided you with some good success afield, and one which you should have learned fairly well by now. If you do feel like you have avanced enough up the learning curve and have developed a new sense of of desire to hunt different places or just different types of hunting then you might well feel the urge to 'upgrade' or 'move up' to a more expensive and better-featured model. That doesn't mean, however, that you will outclass the Classic ID.
Let me remind you that there is no single 'best' detector for all hunting applications. One of the most important tools you can ever have in your arsenal of equipment is a 'back-up' detector. For some, having two models provides them with a 'back-up' in case of field failure with their primary unit.
For others, however, having two models, or three models or four models, goes beyond just having a 'back-up' detector. It really accomplishes having one or more detectors that might use different types and/or sizes of coils and are tailored for a specific assignment. They are at the ready for handling specific challenges.
In your case, I think it would be wise to hang onto a proven performer. This is especially true when you factor in the fact that some of the newer and more featured makes and models lack some ability to hunt in dense iron trash as well as the Classic series.
Your Classic ID has an easily accessable trimmer to adjust the GB that affects BOTH operating modes. The Prizm's trimmer is smaller, more delicate, and doesn't help both mode in the same was as the Classic series.
If hunting a trashy site, the Classic ID allows you to sweep slowly where you can benefit from the quick response and recovery time. Not so with the Prizm's which have a delayed response. The Classic ID is also a little more forgiving so far as sweep speed used.
To top it off, a good, slow-motion discriminator should provide a variable discriminate control to 'fine tune' the rejection of iron nails and such (the Prizm's don't) and you should also be able to use a smaller size coil to work in and among trash. With you Classic ID you have a 4", 6