I agree that this is a good font-pairing but will add that the two typefaces are pretty similar, so you could always just use the different weights of one of the typefaces to improve load times on the website and simplify coding.
I'm guessing your using Lato in this pairing because it's a little less “straightforward” than Open Sans and has a little more personality.
Lato is a little more geometric and playful than Open Sans which gives it a lighter tone to me.
You might want to compare your choices to others, like Hoefler & Frere-Jones do in this blog post. Although you're right in choosing these typefaces for their similar widths, it might be better to create a pairing for several reasons at once if it resonates with the overall design scheme.