Actually, taking a closer look at the Cross Products unit, there's no ICE provided by the unit at all! What it does is remove the M68K chips from the Mega Drive and MegaCD units, and replace them with regular M68K chips on the SNASM board itself, with some extra hardware sitting in the middle, but there's no "In Circuit Emulator" as such. What they've done is simply add logic between the M68K processors and the rest of the system, which is able to snoop on the bus while the processors are running, and halt or intercept bus operations where required. This is enough to give basic debugging functionality, such as the ability to set breakpoints, read and write to memory locations, etc, from the debugger itself, but it's not a full blown emulation of the processor. This is a good idea, as it gives 90% of the features a developer usually wants, at 10% of the price of a full blown ICE unit.
Some of the features the ZAX ICE unit would be able to provide over and above the SNASM system are as follows:
-Logic analyser. This is a pretty powerful feature, especially for debugging timing related problems, or problems related to bus requests.
-Trace functionality, without introducing delays (SNASM does have trace support, but it would probably have to rely on trace interrupts, slowing down code execution dramatically. Not sure though, if their bus monitoring is clever enough, they could avoid using trace interrupts)
-More powerful breakpoint features. I don't think the SNASM system can break on memory read/write at a target address for example, which the ICE unit certainly could.
Again, for more detail, we'd need to compare the SNASM manuals with the ZAX 318P manuals.