It really depends... I have both a Tunze 6025 and 2 of Koralia 4s.
Flow wise, I think the Koralias are much better. There is almost no directionality (real word?) to the pump since it has an open dome cover over it vs. the Tunze Nanostreams which have a slight cylinder to direct flow.
This really allows you to put these pumps a little closer to your corals than you could the Tunzes. I felt build quality was slightly lower or on par with the Tunze nanostreams. The koralias are a bit larger though (the 4s are slightly larger than a 12 oz. soda can) and they are less maneuverable than the nanostreams.
As for why they always start up in the correct direction without a prop stop... I'm pretty sure I know why this occurs and to be truthful, they don't actually start up in the right direction everytime. What happens is that the whole impeller prop assembly is one piece and it slides smoothly along the shaft. The shaft is actually much longer than the impeller prop assembly (roughly an inch longer), so what happens is that when it spins in the wrong direction, the force of the prop pulls the whole assembly out of its ferromagnetic cavity (where the impeller normally resides) and then of course loses its outward thrust and is pulled back into the cavity by the the magnetic force and starts in the right direction (of course, this doesn't always happen the first time so you'll get some start up racket, but I would say it lasts less than 1 second). Of course, sometimes it does start up perfectly in the right direction and you hear almost no sound on start up except for a faint click. (So, technically starting up correctly in the right direction is still 50/50, but it always ends up in the right direction due to the above process)
If people wanted to, they could get the same results with the maxijet mod if they used the same principle. Just make the prop housing much longer and let the impeller prop assembly slide easily along the shaft.
The koralias also have a kind of plastic washer attached to the bottom of the impeller and it sits on an elevated piece of plastic or another washer (possibly ceramic) in the impeller cavity. It's very slick so it may be teflon coated to decrease resistance.
I think they are great pumps and actually probably better than the Tunze nanostreams in terms of type of flow (much wider stream). Pumps are quiet once running and the suction cup mount is a really amazing idea as it prevents slippage and actually reduces the need for a stronger magnet to hold these pumps up.
Cheers,
John