To me, a choice like that depends what you like in a manual wind movement. The 30 mm movements are wider (and with a bigger balance wheel and thus theoretically better accuracy), versus the 6xx series, which are thinner but only 28 mm wide (and thus dressier and typically lower on the wrist, dependent on case reference number). Typically I believe the 30 mm are regarded as slightly more collectable but both are very fine movements. The 30 mm calibres were designed from the ground up as manually wound movements, and the 6xx series were based on the highly regarded 55x and 56x series automatics but without the automatic rotor assembly. If I personally had to choose, I'd go the 28x, because it's 33% thinner but only sacrifices 7% in width. I may be bias though, since I've previously owned a 601 calibre Seamaster 600 (uncertain service history but it kept time to +10 seconds a day consistently in the time I owned it) and presently have a 561 Connie. The larger dimensions of the 30 mm may make it slightly more robust which may be important in an every day watch. Both have an ample amount of spare parts available.
I'm probably rambling. Good luck with your dilemma, they're both very nice choices.
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