I'm gonna go out on a limb here and say that your answer is going to be: It depends.
There's no consistent use of iconography for the "back" function across platforms and devices, so generally the most you can hope to convey with either arrow is "left", which has to translate to "back" by common convention.
The trick is to find the right context so that users understand that meaning implicitly.
I'm going to pick on the left arrow for a minute here by comparing some examples:
In the first example we see a left arrow, which could mean either of the things in the other two panes. I would suggest being explicit with either arrow type in that context.
Continuing with our mobile theme here, let's consider some examples of back arrows that do work:
Here both examples use a chevron, but that's not what's important, is it? The navigation bar gives the icon the "go back/exit" meaning by virtue of convention.
So putting this into a web context, let's look at a good example and a back example, and this time we'll pick on the chevron for once:
Ignoring the atrocious color scheme, here if we examine the arrows the left one could just as easily mean "collapse the sidebar" as "go back a page". The orange arrow offers a little more context that better suggests "go back to the last thing you were looking at". This type of back arrow is even more powerful if it's paired with something like the breadcrumbs in the example that reinforce the context.