1. There is a simple explanation of how the SP gained a more working-class base than other left groups. Its growth was mainly in the Labour Party youth movement in the 1970s and 80s. The Labour Party (then, anyway) was more working-class in composition than movements like that against the Vietnam war. Quite a lot of young people who joined Militant came from "Labour Party families", joined the LPYS because of what they'd learned from their families, and then joined Militant because they wanted something more left-wing and Militant was the visible option.
Since 2000 the SP has focused its effort on street and door-to-door activity in working-class areas, and trade-union activity, utilising the cadres it has retained from the 70s and 80s.
2. Antonio Gramsci once commented on the mechanical Marxism common in the Italian socialist movement in his time that "in its most widespread form as economistic superstition, the philosophy of praxis loses a great part of its capacity for cultural expansion among the top layer of intellectuals..." He did not think that a good thing. I do not think that it is a merit of Militant/ the SP that they can only recruit people with little prior access to books and broad theories, and that it is extremely rare for anyone with any previous breadth of reading to join the SP. (You wonder, of course, why people with some previous breadth of reading join the SWP. I think the answer is that once you accept the snobbish, but common, view that "agitation" lives in a different world from "theory", and almost anything goes for "agitation" so long as it "fits the mood", the SWP looks more serious).
3. Peter Taaffe is an exception to this rule, but generally SP members do appear calmer, less nervy and shrill and "ranting", than SWPers.
I don't think it means that they are "nicer human beings". Most SWPers are congenial and reasonable as individuals. Probably all the groups are much the same in their proportions of affable types, surly grouches, timid say-nothings, and arrogant pontificators. It's a matter of what traits the group's customs accentuate and push to the fore.
The SP's relative calmness does not reflect good customs, or even better customs than the SWP. It reflects a collective culture of immense smugness, complacency, and philistinism (collective culture, I stress: I'm not talking about the personalities of individual rank-and-file SPers). In contrast, the SWP's nerviness at least reflects some unsureness and awareness that things may be questionable.
Look at local anti-cuts committees. The SWP goes off on crazy sidetracks (like their rival anti-cuts committees in Lambeth), and is often obstreperous and uncongenial, but usually it is there. The SP is usually not there, or there only to observe, unless it has local control.
Look at the experience at Vestas.
Recall the 1984-5 miners' strike, and leave aside Liverpool for now. The SWP were slow to join the miners' support groups, but once they joined they worked ok. Militant never joined the miners' support groups. While all other leftists, SWP included, were on the streets collecting for the miners, Militant would go on miners' demonstrations with collecting tins asking for money to launch the promised "daily Militant" (which never arrived).