I'm looking at getting a good general purpose lens and I've had a few people recommend these two lenses but I'm wondering if anyone has opinions on which is better.

At the moment my set up is a Canon 400D with a Canon 35-135 USM lens, nifty-fifty, and Sigma 150-500mm OS lens.

I am after a lens that will provide really good image quality through the general use range. I generally take photos of kids, pets and animal/plant shots with this main lens.