Tamron 18-270mm vs Canon 18-200mm
1) Which one would give me better quality pictures, and which one would have more Zoom?
based on MTF charts, a perfect copy of the 18-270 should provide better resolution than a pefect copy of the 18-200 ... you may or may not find a pefect copy of either lens ... 270mm is more "Zoom" than 200mm
2) Would the Tamron 18-270 have more coverage and zoom than my Canon 18-55&55-250 kit?
18mm is the same "coverage" as 18mm ... 270mm is more "zoom" than 250mm
3) Also, just out of curiosity, would the Canon 18-200mm be a lot like the Tamron 18-200mm?
the canon 18-200 has image stabilization and the tamron 18-200 does not ... the 18-270 does have image stabilization ...