It won't make a lot of difference for casual useage. The Canon EF-S 18-135mm f/3.5-5.6 IS will probably give you better image quality between 18mm and 135mm in terms of geometric distortion, vignetting, etc. They're both about the same in terms of sharpness at common focal lengths and apertures. Obviously the Tamron 18-270mm f/3.5-6.3 Di II VC will give you more reach at the expense of some image quality, but the sharpness of the Tamron above 135mm begins to degrade and by 270mm is fairly soft. Just how usable the extra reach on the Tamron will be is debatable.

Both these lenses will be fairly good when outside in daylight. Neither will be particularly good indoors without an off-camera or bounce flash. Either will allow you to learn the basics of how to operate your camera but neither will allow you much room to explore wider apertures and how they affect depth of field. For about the price difference between the 600D body + Tamron 18-270 and the 600D Body w/18-55 kit lens you will save enough to also buy an EF 50mm f/1.8 II and a generic flash such as the Yongnuo YN-560 II plus a generic off shoe cord or wireless radio trigger. That is what I would recommend. If the primary purpose of the purchase is to allow you to learn the basics of photography, the wide aperture prime will be worth far more to you than a narrow aperture, large focal length range zoom.

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As to the part of your original question regarding which of the two options would be better for shooting weddings:

None of the above. If you are serious about doing weddings you need better gear than either of the options you have included in your question.

You need a body capable of faster handling than the 600D with only one control wheel. The Canon semi-pro and pro bodies include a second wheel on the back of the camera and a higher number of dedicated buttons so that you can adjust many more parameters when shooting without taking your eye away from the viewfinder. While you can do a lot of the same things with a Rebel body, the time it takes to dig through the menu to adjust a setting will often mean the shot has been missed by the time the camera is ready.

You need lenses faster than either of your choices to shoot an indoor event such as most weddings. At a minimum you need a normal zoom with a constant f/2.8 aperture or a set of faster primes such as a 35mm f/1.4, a 50mm f/1.4, and an 80-85mm f/1.8. You also probably need a fast telephoto lens such as a 70-200mm f/2.8.

You also need to remember that you should never shoot a wedding with only one body available. Always have a back up body and enough lenses to cover the job should any one piece of equipment fail. You also need some off camera lights and modifiers for the formal shots.

Just from the perspective of camera body and lens, a typical wedding pro will have something like the following:

• Two bodies capable of fast handling and good high ISO performance. Something like a Canon 5DIII and the 5DII it replaced or maybe a 7D for the telephoto shots if there is enough light for the APS-C sensor.
• A fast normal zoom lens, such as a 24-70mm f/2.8, a fast 50mm prime, and a fast telephoto lens. For backup something like a 24-105mm f/4 will work in a pinch.