Tamron 18-270mm vs Canon 18-200mm vs Canon 18-135mm vs Canon 18-55mm+Canon 55-250mm --Which should I get?

I am upgrading from the Canon Powershot S5IS to a used 50D, my first SLR! :D I like to travel and will be doing so for two months soon, 4 mos next year. I’m thinking an ‘all-in-one’ lens is right for me because being such a n00b I probably don’t want to deal with switching lenses right now and will be better off without the extra weight.

I’m looking at the Canon 18–135mm, 18–200mm, and Tamron 18–270mm. I am able to find them all within about $100 of each other price wise so cost is not the main criteria.

I’ve read the Tamron doesn’t perform great at the extreme ends of its range (soft), has worse lens creep than Canon, and has MUCH slower autofocus than Canon (I love taking action shots), but on the other hand that it outperforms Canon in the mid-range. I read some other stuff, too, but it went over my head. I read the 135mm is a better quality lens than the 200mm, but of course the trade off is 65mm. I also found someone on the internet that was debating between keeping the 18–55mm and 55–250mm or switching to the Tamron; I think he decided to keep all three. I’ve heard the 18–55mm is better quality than the 135mm or 200mm Canon and am now also considering buying the two Canon lenses that other guy had. I did like the sound of the 6 year warranty Tamron offers.

Anyone feel like weighing in on which one I should get?