I am into amateur photography and I am looking for a decent walkaround lens for my canon 6D. What should i prefer between the EF 24-105mm/f3.5-5.6 IS STM and EF 28-135mm/f3.5-5.6 IS USM?
You want the EF 24-105mm f/4L IS (version I or II).
I wouldn't recommend the 28-135 IS because it is not as sharp as the three 24-105 lenses. It isn't available new anymore, but if you are on a budget, a second hand 28-135 can be bought for about $100-$200. Fot the price it is actually a pretty good lens.
All three Canon 24-105 lenses are much better though. The "non-L" STM is quite a bit cheaper than two the L versions, and may meet the needs of the OP.
I still have two EF 28-135mm/f3.5-5.6 IS USM lenses. At their price point they are a no brainer. Best bang for the buck there is pretty much. Not as sharp as the newer current models but you expect that. In general use I doubt you will be ble to tell the difference. If you are a pixel peeper or you intend to blow up your photos a great deal, heavy crops, etc, maybe you should pass on it.
Passing on the bargain lens I would buy and recommend the Canon EF 24-105mm f/4L IS II USM Lens. You might check if this is available in a 'white box' version. Same lens only removed from a kit and sold separately. I don't know if they have them or not but you can try.