Whats the diff. in a 2 circuit and a 3 circuit converted
 

Ok, I see 3 circuit dominators and 2 circuit dominators. And I see 3 circuits converted back to 2 with modified metering blocks. Also throttle stop carbs with the intermediates leaned out.

Is there any advantage to starting with a 3 circuit, converting it back to 2 circuit over just starting with a 2 circuit, like a 8082 1050.

What about 1150 and 1250? Do you need a 3 circuit more the bigger the carb?

Does anyone run 1250 2 circuits?

Mark I have a QFX 1450 body carb with a modified venturi that runs just fine with no intermediate fuel. That carb flows in the 1700 CFM range. 7.56 at 174. I have a 2.400 billet carb that flows over 1800 CFM that goes idle to WOT on a pump gas 540 with no issue and no intermediate fuel.

The intermediate circuit was added originally for individual runner induction, like the manifold in the link below. The reversion pulses that you see when running without a plenum disrupt the signal at the booster at lower RPM's, the intermediate circuit was added to fill the gap.
http://351cleveland.wikifoundry.com/page/SK+45895+

There are instances where an intermediate circuit can be beneficial but require a significant modification, methanol Dominators can benefit as getting sufficient fuel "correctly" through the boosters alone isn't easy, with the right mods the intermediate can add just enough fuel. I have some testing to do, but I think the same mods can be employed on very large carbs, 2.25 blade and over, to help when distribution in the induction is poor. And it doesn't take much to help, the methanol carbs I recently built I had about the same intermediate fuel metering passage sizes that most carb builders use with gas carbs. 4 and 6 cylinder Comp engines can benefit a little as reversion pulses are stronger on those engines. But for most cases there is just no need or benefit and with a lot of the calibrations I've seen using the intermediate is detrimental.

As far as which is best to use? The cheapest one you can find in good condition. A 2 circuit is easiest as you can use the original metering blocks with some minor mods. HP's are a little easier as the air bleeds are already threaded. And I prefer annular boosters, using more smaller discharge holes are the best. BLP sells 16 .052" hole inserts, those are what I prefer.