I haven't compared them head to head, but I wouldn't expect a massive difference. The XPA-200 boasts a marginally more capable power supply (360 vs 300VA transformer, 90,000uf vs 40,000uf worth of secondary capacitance), so in theory it's "better". OTOH, the difference in rated power doesn't amount to a hill of beans, particularly when you keep in mind that the XPA-200 is rated at 1% THD vs 0.1% for the UPA-2.
That said, there are a few nice things about the XPA-200. The connectors are all solid, which was a complaint about some older Emo gear. The gain was also reduced from 32dB to 29dB, which could be useful in some setups.