Hard to say Slash frankly.
With either, the dynamo has to be spinning fast enough to get above battery voltage before it will cut in and charge. At 6v, it should normally kick in around 1000rpm, at 12v somewhere between 1500-1800 at a rough guess - unless the dynamo has been rebuilt with "12v" armature and field windings, when it should behave pretty much as per the 6v set-up.
But quite often, a dynamo will need a good few revs to kick in for the first time, even after quite a short lay-off. And will then behave a bit better for the rest of the time the engine's running. A lot of us have this experience.
I haven't noticed any significant difference in cut-IN speed of the 2 regulators, personally - but the cut OUT on the DVR is cleaner on descending rpm, which eliminates the discharge seen on the ammeter at tickover speeds with some regulators, including mechanical ones.
If you're at 12v and want low revs cut-in, not much for it but to fit the finer 12v windings to be honest.
This all assumes the dyn is in good nick of course, and doing what it's meant to. A dirty commutator, damaged brushes or poor connections will cause a reduction in performance.
(You could always stick a meter between a wire linking D and F at the dynamo and a good earth, start her up and then see (tacho) or guess how many rpm are needed to get the output above 12v to give you a clue. Anything over say 2000rpm wouldn't be good.)
I use DVRs at 12v with original Lucas spec windings myself. That's OK because I live in the boondocks and don't have to tootle around at low revs with big wattage lights on. These days I use LEDS on dynamo bikes, so it doesn't matter any more anyway. But if you want low rpm cut-in regardless, you either need to run at 6v or get those 12v windings.
