Interesting to me, therefore, was only the question of which is the better of the travel zooms currently offered? I have therefore the "Canon EF-S 18-200mm f / 3.5-5.6 IS" and "Tamron AF18-270mm 3.5 / 6.3 Di II VC" extensively tested (test panels and landscape) and both lenses compared.
The result is - for me anyway - clear: The winner is Canon! Particularly with regard to sharpness and contrast this is better than just deliverable Tamron over the entire focal length range.
Only in the case of the vignetting Canon is weakening in the area over 100mm. This vignetting in the corners is clearly visible at maximum aperture 5.6. When dimming to 7.1 it is noticeably smaller. Only in this discipline, the Tamron is better, although there but also in the high zoom range visible shadowing shows, but it is lower.
Color fringing (chromatic aberration) occur naturally, as with all travel zoom in the screen area. Even when Canon lens which is recognizable and not to reduce by stopping down. However, they occur only in the horizontal direction, while the Tamron produces both horizontal as well as vertical fringes.
The distortions are similar to those of other mega-zooms, ie 18mm thick barrel-shaped, from 35mm then slightly cushion-shaped. For architectural photography in short focal range so a lens changes may be required, or a treatment with an image editing program.
Conclusion: As a universal or travel zoom is the "Canon EF-S 18-200mm f / 3.5-5.6 IS" well suited. It provides sharp and high contrast images throughout the focal range. Optimal is a stopping down by one or two stages. Then the vignetting in the area over 100mm, which could interfere with landscapes disappears. For heavily zoomed persons or wildlife shots this fall anyway not.
Focusing work reliably, the image stabilizer is a great thing.
The "Tamron AF18-270mm 3.5 / 6.3 Di II VC" has a higher zoom range and quite acceptable levels. However, cuts in my tests (on a Canon 40D) underperformed. And one can also say that it is a class better than the "old" Tamron Mega Zoom (without stabilizer), the "AF18-200 4.5 / 6.3 XR Di II".
But: Both lenses were tested by me on a Canon 40D. Only applies for this review. It may be that the Tamron Nikon supplies a better results. Recent reviews seem to confirm this.
Unfortunately, it must be mentioned for Tamron 18-270mm still something negative. Designers is a mishap happens. If you hold the zoom down - For example, to photograph an object close to the ground - so blocked the zoom ring. No more bets, the zoom range can no longer be in this position adjust. Actually, an absurdity. Since a repair is necessary and hopefully possible.