I'd go the non-GMT version of the Seamaster, (the GMT honestly is too busy and messy, the non-GMT that you're going for is really sharp). Its a solid movement, a very tough case, and suitable for all weather. The Speedmaster is a Cal 3313 which is not a bad movement or anything but I'd rather then 3330 in the Seamaster.
The Seamaster also has a nice party trick in that its pushers are waterproof even when being used to full diving depth, the vast majority of chronographs including the Speedmaster are not, and if you use the pushers underwater even while swimming you can cause water ingress, the Seamaster Chronographs since 1993 have been designed to withstand that.
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