Hi - I had 2 copies of the 18-135 STM, both used on a 100D/SL1. The first developed CA in the centre of the image field (?an IS issue?), and went back, the second was just fine and was my main travel lens for 2 years, in Europe and Vietnam/Cambodia. I gave it to my daughter and she uses it on her 1100d, including some nice wildlife images from safari in South Africa. Neither of these are high end cameras, but when I compare results to my 5D3 and L lenses, it holds up very well.
However I have now had the USM version for 2 years and while it is said to be optically identical, I do find it just a touch sharper, used on an 80D as my travel kit. It definitely does focus faster on this body, but I can't compare it on a 100D/SL1. The USM version is slightly heavier but looks and feels more robust, only time will tell, it's not weather sealed, but I look after gear carefully.
Either will serve you well as a general purpose walk around lens. Both are best in the middle of the zoom range, and not quite so good at 18mm or 135mm, these are best at f8 in my experience. Unlike many lenses, neither of mine have suffered from the dreaded "zoom creep", which is a bonus. You will be very happy with the image quality. Do buy a lens hood. On balance I would go for the USM version.
If you need wider, consider the 10-16 STM which is sharp, light, and inexpensive.
If you need longer, consider the 55-250 STM which is also sharp, light, and inexpensive!