I have a SON Delux on the Streetmachine and a Shutter Precision SD-8 (6-bolt) on the Baron. They're functionally identical (though I've barely done 2000km on the Baron, and mostly in dry weather, so hardly a long-term test of reliability). The SON Delux is Centerlock, so I had to use an adaptor to fit the 6-bolt[1] rotor (which also means you need a lockring tool to get at the spokes). SON has spade connectors, vs the Shimano-style lego brick of the SP (I prefer the lego brick for roadside serviceability, but it's a minor concern). The SON is better looking, and has Schmidt-quality engineering and a price tag to match.
There's also the Shimano Capreo hub, which I have on the Brompton. Very much a budget option (akin to the DH-3 series), and seems quite draggy. Would only recommend it as an alternative to battery lights, or for a utility bike that isn't being ridden far enough to justify the cost.
I've used a SON28 (designed for full sized wheels) in a 20" wheel before. It works fine (indeed, you get a slightly increased output at low speed), but it's not an optimal arrangement (adds weight and drag), and something I'd only consider if it was parts that you had lying around.
[1] Yes, you could use a Centerlock rotor, but in the interests of compatibility, I've standardised on 6-bolt.
