I've never owned the Tomahawk but spent quite a bit of quality time with it and my UM2s at a recent meet. I haven't heard any of the latest Xin amps but was a regular customer of Xin's beginning with a very early version of the SuperMini up until the 2nd or 3rd version of the SuperMacro. I love the sound of the Tomahawk and always enjoyed the Xin products I've owned. At this point I don't own either a Xin or RSA amp so take what I say with a grain of salt, and please, don't think I'm flaming or endording either product. I'm just stating my observations.
Sound quality aside, I would go with the RSA amp. The fit and finish of the Xin amps I've had always seemed a bit rough, whereas RSA amps are always professionally built and are examples of true craftsmanship. When the Xin SuperMini was under $100 with a student discount the Xin amps represented a tremendous bargain. Now that they're over $300 I'm not sure they are up to snuff.
On top of that, it seems like when Xin releases a new version of his amps it almost renders his older version obsolete. He sometimes offers an upgrad path, but not always, and it involves sending your amp in for service. I'm not quite sure why, but when a new version of a Xin amp comes out, it causes the previous versions to depreciate faster (well I know why but it's not really that logical - the new version doesn't make the old version sound any worse).
On the other hand, once RSA releases an amp it is a finished product and does not require any newer version or modification. RSA's first "portable" amp, the XP7, is still in production and still has its place. This helps the products maintain their resale value.
In terms of sound quality I think both products have a lot to offer. My previous SuperMacro has a very immedate and impactful sound. The new versions are said to have the "tube-alike" sound but I haven't heard them. The Tomahawk also seems to be a very upfront amp, whereas the Hornet and SR-71 seem to be a bit more expansive in sound with bigger soundstage. From my memory, the Tomahawk and SuperMacro seemed fairly similar in presentation.
The difference between the Xin and RSA amps is more a matter of taste more than one being better than the other. If you're not too concerned with the possibility of your amp being considered obsolete in a relatively short period of time, I don't think you can go wrong with either amp. I'd also go with something you've heard and liked if at all possible. Auditions are always key and no two sets of ears are the same.