I have not looked into those options Kym, I will though, thanks.

I am not the biggest fan of my current lens. Obviously, yes the range is fantastic, however the quality is not. The Af is quite slow, and sometimes often inaccurate, lots of distortion and vignetting, doesn't seem too sharp, and I'd like a constant aperture.
The practicality will be different to what I am used to yes for sure. However I am finding myself switching my lens between my tamron and my 50mm f1.8, and do not find this annoying at all, so the practicality of having to switch a lens to get more zoom does not phase me. THe new lens is not only for the trip to South America, that's just where it will get it's first proper use. I plan on using it as part of an ever improving kit for years to come.
I do plan on taking my other lens, simply for the extra zoom though. Although, ideally I will have another new lens to go along with this one (undecided on that too, 70-200, 7-300, 100-400) however I do not think that will happen in time for this trip unfortunately.