NGK CR8EK is a twin ground electrode plug; CR8E is a single overhead ground electrode. Or at least they should be according to the catalogues.
They're both a pretty basic plug and you can (at least in theory) get better performance from a fine wire Iridium plug. There are so many influencing factors though, it's very difficult to say if you'll see a benefit or not.
A recommended Ir plug is the CR8EIX. For me they work well.