While I do not have the S600/S620, I do have several regulators that are as small or smaller than the S620. A Titan Supreme, Core Supreme and now a Jetstream and a couple of others. I do not notice any difference when used on a necklace. Now, a Conshelf I notice on a necklace and a 109 as well because they are a bit heavier in the water and tend to hang so I tighten the bungee length up a little more, probably a good idea anyways. Since adding a light to the top of my Nauticam housing I notice that it bangs into my necklaced regulator, enough that I am considering going to an Air 2 (!!!!) and I am being halfway serious. Even without the light when I raise the housing up to use the viewfinder (vs the LCD screen) it also bangs into the necklaced regulator. I do not think a sliglty smaller regulator would cure this issue. I am mostly diving G250s of which a G260 is a sleeker and slightly smaller footprint housing the same goodness as the G250 on the inside.
Late to the G250 party, I am not sure there has been any noteworthy advance in second stage regulators since the G250 came on the scene in about 1980 and still in production for Halycon and really the G260 is a G250V.
But again, I would not turn down a S620Ti. Had my LDS offered me a smoking deal and presented me the choice of a S620Ti or a G260 what would I have done. I do have a lust for titanium, poor old Jimma Carter. But, no, he only offered me the smoking deal on a G260 set, ha. Poor me
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Rambling, I know, but, I do not think the minor difference in size between the S620 and G260 will be noticeable on a necklace.
James