Welcome to the TDP forums!


I'll cast another vote for the EF-S 17-55mm f/2.8 IS. Personally, I have both that lens and theEF 24-105mm f/4<span style="color: red;"]LIS USM. I use the 24-105mm as my outdoor walkaround lens in inclement weather (on a 7D, which isweather-sealed like the 24-105mm, whereas the 17-55mm is not). But the 17-55mm is more versatile - 24mm is just not wide enough in many situations, and although you have the 10-22mm (as do I), it's not always convenient to switch.


If I had to keep one of the two, it would be the 17-55mm. For travel, the 17-55mm + a 70-200mm L zoom is a great combo.


As others have stated, you cannot use the Canon 1.4x extender with the 24-105mm lens (of your current lenses it works only with the 70-200mm f/4L). While you might be able to use a 3rd party extender, the resulting f/5.6 zoom would perform worse than your 18-135mm.


Regarding the 50mm f/1.8, it's a nice little lens, with a cheap feel. The f/1.8 aperture will help indoors, certainly, but to be honest you may get better results with the 17-55mm f/2.8 combined with flash. Avoid the pop-up flash like the plague (except perhaps for moderate fill-flash outdoors), but an external Speedlite flash bounced off the ceiling can give very good results - for that, I'd recommend the 430EX II. That one (but not the cheaper 270EX) will really help out with low-light focusing (something the 50mm f/1.8 can struggle with).


Good luck with your decision!