I've got both a Onix SP3 and the slightly Lower End Yaqin MC-10L, I'm really happy with both. Some general observations - the Onix/Melody amp seems to be better built, there is a lot of info on what people prefer for tube rolling and bias levels. However it only has 2 inputs.
Some of the Yaqin materials are more cosmetic than functional, and the chasis isn't as beefy as the Onix. You do get the ability to switch between triode and ultra-linear, and you also get the ability to bypass the pre-amp section and run it as a power amp.
Lately they've had a few SP3's at AV123 for around $399 at that price I would go for it. Otherwise I'd probably go for the Yaqin, but new they usually go for around $699. You could probably get a better deal on the Yaqin used, since they probably don't hold their value as well.
The other factor is how big a deal is having a warranty and service to you - the Onix is way better in that aspect. Although supposedly AV123 is switching from Melody amps to Xindak amps, so support may be more spotty in the future with Onix.
Either way I think it would be hard to make a mistake with either of these.
Jim C