Pretty much since he announced, I have been planning to vote for Bernie in the primaries. I could go into a long analysis of why...but it actually comes down to something very simple. Bernie will regularize and validate the far left of the political spectrum. Though this may sound strange, it all basically boils down to rhetoric for me.
I begin with two basic assumptions (valid ones I think, but still assumptions)
1. Bernie is more progressive than Hillary...and he shouts it to the rafters.
2. The Republicans are going to retain control of the House of Representatives, and maybe the Senate.
What this mean is that in terms of actual legislation, whatever the differences between Hillary and Bernie are, neither of them will be able to achieve all of their campaign promises. Neither Bernie nor Hillary will be able to accomplish their whole agenda (Bernie would be able to accomplish less, merely because he is proposing to do more). In fact, what they can actually accomplish in terms of actual legislation is pretty-much-the-same.
So, both will govern in much the same way, whatever their promises are now. What this leaves then, are two issues.
1. Electability
2. Rhetoric
Electability is the primary issue that Hillary supporters can legitmately point to in favor of their candidate. My rule of thumb is that I vote for the most progressive candidate who can win the general. I think Bernie can win the general election, but at this point I cannot point to anything to back up that conviction.
That said, at this point, it really does look like Hillary will do better against the republicans than Bernie will. That's to be expected. She is more conservative than Bernie is, so we would expect more moderates to break her way than they would Bernie (all else being equal, which of course it isn't). All that said, we are still a long way off, and the polls are not that reliable yet. If, in the fall/winter next year, it looks like Bernie cannot defeat the likely republican nominee, I will re-evaluate my vote in the primary. As I said, I vote for the most progressive candidate who can win.
But the reason I am truly voting for Bernie actually has more to with rhetoric than policy, since (as I explained above) both Bernie and Hillary will be similarly constrained by the Republican controlled House. If Bernie wins or does well in the primary, the commie-pinko-socialist wing of the Democratic Party will be become more normal in the eyes of most Americans generally, and the democrats specifically. Simply put, if we can demonstrate our numbers and strength, the democrats are more likely to take the recommendations of the progressive wing of the democrats more seriously.
So, despite all the policy differences between Hillary and Bernie, despite the simple fact (IMHO) that Bernie is obviously more progressive than Hillary...those are not the reasons I am voting for him in the primary. Really it comes down to this...I'm voting for Bernie because he will loudly, intelligently, and doggedly champion progressive values--legitimizing and normalizing progressive values. With luck, that will serve as a foundation to further move our country toward a more just society.
PS The diagram above also explains why I will vote for whatever democrat is nominated for the general election. Any democrat will make this country somewhat more progressive, any republican will do the opposite. Not voting for the Democratic nominee, when a republican president would have the House, and very likely the Senate is batshit fucking insane