It depends a little, but pretty much lean towards swivel.
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If it has two swivel locks and they are stout, like in the pic of my Reed above - than a swivel all the way. The swivel lock handles are around 7" long and they tighten down the big locks well beyond what I could ever physically push it to.
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This Hollands on the other hand, only has one swivel lock, and it feels no where near as secure as the Reed.
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This Parker is a smaller 6" like the Hollands, but it has two swivel locks. Each swivel lock is smaller than what the Hollands has, but feels decidedly more secure.
From my experience, if it only has one swivel lock, I'd probably prefer a stationary. Dual locks? The two locks holding it from swiveling is just about as strong as the 4 bolts holding it on the bench, so it's not much of a concern for me. Swivel all the way!