Which camera is best for you and your needs depends on a couple things. If you want easy access to more of the settings and controls because you will be changing them on the fly, a more accurate and/or more sophisticated autofocus system (if you plan to take control of it), faster continuous shooting speed, and some additional abilities to customize the camera's controls and functions to work exactly how you like, then you may want to step up from the T2i or T3i (pretty much the same but with rotating screen) to the Canon 60D or even the Canon 7D. On the Nikon side, this means going from the D5100 to the D90 (an older but cheaper model) or the more current D7000.
If you like to "keep things simple" as you say, you may want to stick with the 60D or D90 and not the more versatile but more complicated 7D and D7000.
For myself, when traveling, those above features are pretty much exactly what I have found to be important in order to be able to capture the shots I want (the autofocus system, the shooting speed, access to more controls).
And of course as you realize, a good quality lens is very important. Determine what focal range you like working in, as you already indicate, and get the best lens you can afford with that range. That Sigma or similar Tamron should be a nice lens, and Canon makes some great f/2.8 lenses in that range too.
So, in order to compare those cameras - the T3i vs 60D vs 7D, have a look at this article of mine which describes their differences and explains why they may or may not be important to you:
http://blog.dojoklo.com/2011/02/20/canon-t3i-600d-vs-t2i-550d-vs-60d-vs-7d-etc/</a>
There is a similar Nikon article also - D5100 vs D90 vs D7000
http://blog.dojoklo.com/2011/04/14/nikon-d5100-vs-d7000-vs-d90-etc/</a>
And as far as accessories and additional stuff that you need or which may come in handy while traveling, have a look at
Accessories for Travel Photography
And of course, learn the camera as best you can before you go - learn to take control of the autofocus system, aperture priority mode, and maybe even the different metering modes so that you know what to do and how to change the settings for whatever situations you run into.