Budget? These two lens may be similar in the focal length range but there are big differences too.
1) Price
2) 18-135 is variable aperture, 24-105 is fixed f4
3) Build quality and image quality (24-105 is better)
4) Weight
5) 18-135 covers wider and longer focal length as it is designed primary for cropped camera like yours. 24-105 is designed more for full frame camera. If used on 600D then the widest equivalent field of vision on 24-105 is actually 24 x 1.6 compared to 18-135 which is 18 x 1.6.
Depending on your budget and purpose, it is not easy to tell you which we recommend unless you provide more details. Usually for new users I would probably tell them get a 2nd hand 18-135 and play with it first later then sell at not too big price loss when they want to upgrade. If buying 2nd hand 18-135 I would recommend used 18-200 instead.
If you are familiar with your camera already and have been shooting for some time, then 24-105 is a nice kit to invest in and use for long term.