I'm going traveling for a few months and I am planning to take my Canon 750d with me, I currently have a 10-18mm, 50mm 1.8, 55-250mm and a 18-55mm lenses. I am thinking of getting a 24-105 L lens to take with me, however I was unsure wether this would be worth it on the camera. I am also debating getting an upgrade to either a 70D or a 80D. I am unsure wether to get a new body or the new lense as I can only afford one of these options. Any help would be appreciated.
The easy answer is, you'll know what you need when you are ready for it. For example, a mechanic doesn't go out and look for new wrenches, he runs into a new vehicle make or model that his current wrench doesn't handle and then buys what is needed.
If I was traveling for a few months I certainly wouldn't be looking for something that bulky and heavy. Your lenses and body right now are relatively light, but when you start talking about bigger glass and bodies made of magnesium - I'd be concerned. Rather I'd look towards a mirrorless or advanced compact with a large sensor.
The EF 24-105mm f/4 L is a very good general purpose lens when paired with a full frame camera. It provides a range of angles of view that make it very versatile in that combination. 24-105mm is much less useful for such a role when paired with an APS-C camera that gives it the full frame equivalent of a 40-170mm angle of view. With the 24-105mm on a crop body most users find the wide end just isn't wide enough to be useful as a "walk around" lens.
To get a lens to do what the 24-105mm does on a FF camera, you need about a 15-65mm focal length with an APS-C body. In terms of angle of view and image quality as well as constant aperture the EF-S 17-55mm f/2.8 IS comes closer than anything else currently offered by Canon. The image quality, if anything, is better with the 17-55mm than the 24-105mm and the aperture is one stop faster, which can help make up for the stop you lose in terms of depth of field and low light performance when comparing an APS-C sensor to a full frame sensor. In the U.S. both lenses currently sell for roughly the same price.
Beyond all of that, though, unless you can articulate exactly what it is you hope to improve by getting a new lens or a new body you're pretty much just trying to find a problem to fit the solution of throwing money at buying new gear and somehow hoping it will improve your photos.
Disclosure: I've shot more sessions with the EF 24-105mm lens than any other lens since I got one six years ago. Other than a few test shots done to perform AFMA matching it to my APS-C bodies (just in case of a future emergency caused by failure of another lens or body), I've never shot a single frame with it mounted on an APS-C camera. If I'm looking for a normal zoom for an APS-C body I'm reaching for my Tamron SP 17-50mm f/2.8 Di II. It is smaller, lighter, one stop faster, and provides the angles of view needed for a single lens solution with an APS-C body.
If it was me I'd be going for the 24-105 L. Its a good high quality general purpose lens to carry around on your camera all day and takes really nice photos (we have one in the family that gets used on a crop and full frame camera.
While not the widest angle on a crop sensor you can recompose with your feet.