you don't need to watch the original dub for much longer as Viz Media has been re-releasing the original series (as well as the new Crystal remake) which includes the final season
Nearly two years ago, we announced the series by surprise at Anime Central in Chicago, IL. For the first time, all 200 episodes, specials and movies would be coming out in English! Since then, we have been releasing subtitled episodes every week. Just point your eyeballs here at VIZ.com for buckets of Moon Kingdom goodness. For fans of the all-new, uncut English dub, both season 1 and season 2 have been released on dual language Blu-ray and DVD. You can find the sets in all their shimmery glory at your favorite anime retailer, such as Right Stuf and Amazon!
Source: Viz Media
looking at the list on wikipedia we see that Sailor Moon Sailor Stars, the last season that the original US Dub never covered in english, goes to episode 200 as such this means that Viz isn't just going to stop at Sailor Moon SuperS (which is episode 166)
currently (27/07/2017) Viz Media sells up to Sailor Moon S, Season 3, Part 2 which is episodes 109-127 so you currently don't have the final season and will need to wait, however it should also be noted that this Viz Media release isn't like the original US Dub release which. as you can see in the list of episodes on wikipedia, skipped episodes in the first season (remember that 200 episodes is the number of episodes from Japan, if Viz was going all 200 they wont be skipping like original US Dub did)
While i have yet to see it to confirm from my understand this also means some of the themes from anime which were toned down/altered in the original US Dub are kept to how they were in Japan. noticeably during the Death Busters Arc (Season 3) the theme of sacrifice and ofcause, Haruka and Michiru
One of the controversies surrounding Sailor Moon S is in the romantic-lesbian relationship of Sailor Uranus and Sailor Neptune, who are subtly depicted as romantic lovers, although this is never mentioned directly. Takeuchi has openly admitted they are a romantic couple, and their voice actresses were instructed to play the characters "as if they are a married romantic couple." However, because of differing cultural standards, most countries outside Japan, excluding Greece, Germany, Sweden, Spain and Mexico, have censored the relationship. Alternatively, in the Cloverway/Optimum English-dubbed version, this subtext was made more taboo by way of portraying Neptune and Uranus as biological cousins who were involved in a romantic relationship with each other. The Studiopolis/Viz Media English-language dub preserves the subtext of the original material.
Source: List of Sailor Moon S episodes
As Viz Media's blog post says that the re-release is duel language i assume it would contain both the japanese dubs with english subs and the english dub. considering that to my knowledge the final season was never released in english outside Japan officially watching the original subbed probably means watching a fan subbed version which you shouldn't really be doing when the series has been officially subtitled.
in case if i am wrong and:
- there is an official release of the final season before Viz Media
- you decided to wait for Viz Media to release theirs
- you want to watch it subbed rather than dubbed
then which subbed version you watch is a matter of personal preference. however Viz Media's release would be easier to acquire as aside from DVD/Bluray release the Viz Media link i posted on Sailor Moon S, Season 3, Part 2 lists digital distributions like Steam and the Playstation Store. i would suspect ADV Film's home release it out of print thus harder to get