^Most fans seem to prefer the American dub but your mileage may vary.
Toho's international dub (originally titled Ebirah, Horror of the Deep but now called Godzilla vs. The Sea Monster on DVD) was produced in Tokyo by Frontier Enterprises and it can be heard on the Sony DVD and the Kraken DVD/Blu. I don't think this one is nearly as bad as most fans do but I still think it's vastly inferior to the second English version in about every way. That one, titled Godzilla versus the Sea Monster, was recorded at Titan Productions (New York) for the original American release by Walter-Reade Organization. The dialogue, the performances, and the lip sync are much better here than in the international version, however, unlike the international version, WRO cut around five minutes. (I don't think it really adversely affects the film.) This version unfortunately isn't available on DVD.
Shameless plug: I edit and upload dub comparisons to my Daily Motion channel. Ebirah/Sea Monster is one of the few I haven't made yet (I'm waiting on a superior audio source for the American version), but below is my comparison video for Son of Godzilla. Like Sea Monster, the international and American versions were produced by Frontier Enterprises and Titan Productions, respectively. The casts for each version are nearly identical to the casts in both crews' Sea Monster dub so this should give you a idea what you'll be getting from the earlier film.
Partial voice credits for Son of Godzilla:
Burr Middleton as Maki (Akira Kubo)
William Ross as Dr. Kusumi (Tadao Takashima)
Peter Fernandez as Maki (Akira Kubo)
Jack Curtis as Dr. Kusumi (Tadao Takashima)
Lucy Martin as Riko (Beverly Maeda)
Gilbert Mack as Fujisaki (Akihiko Hirata)
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