Personally, I'd watch the original DB, then switch to DBK. DBK is a trimmer version of DBZ - and so far as it's been dubbed, it's a better dub than the DBZ dub.
There are fortunately not many changes in terms between the Japanese and English versions, and most of the differences between the R1 subs and the dub are a matter of orthography ("Freeza" vs. "Frieza" for example).