I don't have anything to contribute on the Jamo end. However, the X-LS products are quite good. Built like tanks. Solid. Plus, the newer "EX" model is in the works which will provide for a revision to the tweeter and crossover. Some of us want a little more detail for music purposes.
The Jamo on the other end, would have to be accepted "as-is". You'd have to love the sound if you wanted to keep them long term. The Onix offers lots of upgrade paths - Ninja crossovers, new cabinet damping materials, etc. The future is nearly endless.
Were you looking to use the system mainly for music or movies?