My sister and I will travel together in Austria, Czech Republic and Germany in September. She will bring a 6D and a Tamron 28-75mm f/2.8 lens. I'll bring a 5D but I'm not sure which lenses to bring. 17-40L is a must, the 70-200 IS II isn't really needed I guess. But I'm not sure if I shall bring the Tamron 28-75 or the Sigma 50 + 100mm f/2 since my sister will bring the 28-75 anyway? The reason why I'm considering the Sigma 50 is for low light shots as a night time walk around lens. But since the 6D high ISO is supposedly excellent, maybe the Tamron 28-75 f/2.8 can do a decent job for this as well? Also do I need something over 75mm focal length? The 100mm f/2 is quite light itself so the weight isn't too much of an issue. Any suggestion please?