Hi royed welcome to the forum.
The right choice of lens can be a complex subject. Much depends on your budget, application and how critical you are of image quality. What would help is to tell us what subjects you want to photograph and the amount of enlargement your photos are printed to.
Why do you want more reach ?
Can you not move closer to the subject ?
The 18 270 range is regarded as a ideal travel lens where getting a shot is more important than critical image quality. If you only use the whole frame to print postcard size prints it is generally good enough. It tries to be all things for everybody (i.e a one lens solution) and full of compromise in design.
Canon make several 70 300 lens so which one are you referring to ?
A general rule of thumb regarding lens image quality if you are using larger print enlargements. A lens with a smaller zoom range will have less design compromises than a lens with a large zoom range.
Things to look out for when comparing lens.
Barrel and pincushion distortion. These tend to show at either end of the zoom range which can be quite annoying when photographing buildings or subject matter with straight lines. Normally best in the middle of the zoom range.
Colour fringing on the edge and corners of the frame. Again the degree can vary at different points in the zoom range on high contrast subjects.
Image sharpness on the edge and corners of the frame. Often improves when smaller f-stops are used. If your using the lens in low light wide open it may be a factor.
Overall build quality ( ruggedness) and image sharpness is mainly a factor of price.
Auto focus speed. Some lens focus quicker than others. Could be important for fast moving subjects.
Weight. The better quality lens tend to be heavier but then do you want to lug the weight when travelling.
Focus distance range. Is getting very close to the subject important.
A good lens test and review website is
http://www.photozone.de/canon-eos
At the end of the day it will depend on your budget, subject matter and acceptable image quality for the size of enlargement you require.
Sadly there is not the perfect lens for all situations regardless of price. It is the case of picking the best lens for the type of working you are doing. Hence the reason why we have number of lens to cover a range of topics.