I believe the only real difference between the Classic ID and the Classic 5ID is the operating frequency. They have the same display and the same two operating knobs and pinpoint trigger switch.
The Classic ID operates at 6.59 kHz. This is the same operating frequency as the XLT, XL Pro, QXT, Classic III, IDX and IDX Pro. The Classic ID can use the same wide range of search coils as all of those other machines.
The Classic 5ID does not operate at the same frequency as the other Classic machines. I believe it operates at 8kHz, the same frequency as the Prizm machines. This limits the available sizes of usable search coils to a smaller selection.
Staying in the same category, a better detector choice than either of these machines would be the IDX or IDX Pro. They both have a Normal/Black Sand switch and a Frequency Offset switch that allows reduction/elimination of interference from nearby detectors or other EMI sources. And, again they can use all of the wide range of 6.59 kHz search coils.
Another possible plus for staying with a 6.59 kHz detector is that a Sunray probe is made for this frequency. There is no Sunray probe made for the 8 kHz machines.
Sorry if I have rambled too much.
Bob