Sign convention for the Minkowski metric $\eta_{\mu\nu}$ In special relativity, the proper time is defined as 
$$d\tau^2 = c^2t^2-(x^2+y^2+z^2).$$
One usually introduces a matrix $\eta$ to represent it. I have seen two sign conventions. One has three minuses:
$$\eta_3=\left(\begin{array}{cccc}
1 & 0 & 0 & 0 \\
0 & -1 & 0 & 0 \\
0 & 0 & -1 & 0 \\
0 & 0 & 0 & -1
\end{array}\right),$$
and the other one has only one, so $\eta_1=-\eta_3$. Thus, $\tau^2=x^\mu\eta_{3,\mu\nu}x^\nu=-x^\mu\eta_{1,\mu\nu}x^\nu$, summing over repeated indices. 
I have been told this is an issue in the physics community, and that relativity typically uses the $(-,+,+,+)$ East coast sign convention while particle physics/quantum field theory often uses the $(+,-,-,-)$ West coast sign convention. Why is that?
 A: Relativists tend to use the proper time, $d\tau$, and the proper distance, $ds$, interchangably. If you're working with proper time you'd expect the equation for it to look like:
$$ d\tau^2 = dt^2 + \text{other terms} $$
while if you're working with proper distance you expect:
$$ ds^2 = dx^2 + dy^2 + dz^2 + \text{other terms} $$
The sign problem comes about because the spacetime signature requires that $ds^2 = -c^2d\tau^2 $. So you end up with:
$$ d\tau^2 = -\frac{ds^2}{c^2} = dt^2 - \tfrac{1}{c^2}\left( dx^2 + dy^2 + dz^2 \right) $$
or:
$$ ds^2 = -c^2d\tau^2 = -c^2dt^2 + dx^2 + dy^2 + dz^2 $$
Both describe identical physics, so which you use is just personal preference.
A: 
Could you provide a simple reason for these two conventions?

The reason behind the (-,+,+,+) convention (the "mostly plus metric") is that a positive length in 3 dimensional space (e.g., the distance from my head to my toes) should still be a positive length in 4 dimensional space-time. Why should the distance from my head to my toes all of a sudden become negative just because I begin to consider special relativity?!
The reason behind the (+,-,-,-) convention (the "mostly minus metric") is that... well... There is no good reason. Clearly, you can see what side of the debate I am on.
Also, an argument from authority: Schwinger preferred the mostly-plus metric. Everybody cool uses the mostly-plus metric. For example Wald says: "We choose to use the metric signature - + + + because it is generally much more convenient than the alternative choice + - - - in that it induces a positive definite (rather than negative definite) metric on spacelike hypersurfaces."
Also, when you decide to look at thermal field theory or when you decide to wick-rotate to a euclidean field theory, if you are using the mostly-plus metric everything works out more smoothly. Whereas if you are using the mostly-minus metric everything ends up having a stupid minus sign introduced for no reason.
A: I think it depends on what you are interested in. For example, in my work considering time dependent fields in field theory coupled with gravity like inflation or quintessence, in the +--- signature you obtain mostly positive terms, intead of having to work with negative kinetic terms all the time. So do as you wish.
