Agreed with the weight ratio... but overall which is better?
The significant differences are the 4mm on the long end and faster aperture of the 10-22mm, vs. the IS and lower cost of the 10-18mm.
To answer the OP's question, based solely on Canon's published MTF charts, the 10-18mm is slightly better than the 10-22mm (and the EF-M 11-22mm is better than both). The differences are minor enough that they would likely not be very evident in real-world shooting.
Also, bear in mind that those MTF curves are theoretical – they're calculated by computer based on the optical formulae of the lenses, not measured from actual lenses. In other words, Canon's MTF curves represent the best possible case, and for real lenses variances in production may affect performance. The 10-18mm has a plastic bayonet mount, suggesting that the production might not be as tightly controlled (but we really can't know).
More importantly, the MTF curves show sharpness and contrast, but tell us nothing about vignetting, distortion, flare control, or any of the host of other factors that impact image quality. Then there are other things like AF speed and reliability, full time manual focusing, handling, etc., any or all of which can be important in determining what one thinks is 'best'.
So, overall which is better will depend entirely on your personal needs.
In early real world tests the differences actually do appear to be significantly in favor of the 10-18. And it costs way less and has IS. Yeah the 10-22 is a faster lens, but how important is that for most in an UWA? I bet not nearly as much as the much better optics of the new lens, the IS of the new lens, and much lower price of the new lens.