Or, patriotism raises it’s ugly head.
How does the European Space Agency’s orbital telescope compare to Hubbel?
The ESA’s XMM-Newton space telescope doesn’t really compare at all to NASA’s Hubble–the XMM-Newton is an X-ray telescope, while the Hubble works in ultraviolet, visible light, and infrared wavelenghts. The XMM-Newton might compare more directly to NASA’s Chandra X-ray Observatory. Here’s a comparison of Chandra with other X-ray observatory satellites, include the XMM-Newton. Chandra’s aperture diameter is bigger–whoo-hoo!–but I don’t really know how they stack up in terms of, ahem, actual performance; they may in fact have different missions. Perhaps The Bad Astronomer or one of our other astrophysically minded posters could provide more details.
Both the Chandra and the XMM (Newton) are pretty impressive instruments, and they are complementary. Chandra has a 0.5-arcsecond resolution, far better than the 20 arcsec resolution of the XMM. So Chandra can take much shaper pictures, allowing us to examine complex structures and separate nearby stars. (Chandra images also look much better on the evening news 
) On the other hand, XMM has a larger collecting area, so it can observe fainter targets in less time. This is important - many cosmic X-ray sources are so faint that it takes hours to gather enough light to make a picture.
I think this trend is common in other astronomical projects. Americans want the shapest looking pictures, while scientists in other countries place more importance on other quantities that can be observed (collecting area, spectral resolution, etc.)