I'm guessing the key to Independents is that they turn out to vote at a lower percentage. There are more Democrats than Republicans. If there are notably more Democratic leaning Independents, then the key to Democrats winning is getting an inspirational candidate. Both Hillary and Bernie can fit that, but only Bernie can fit it for policy reasons.
As basically everyone here can tell from my facebook page, I'm 100% behind Bernie Sanders this election. I feel that if he manages to get the Democratic nomination, he will have beaten a far tougher foe than anyone the the Republican side has to offer. As such, winning the primary will show his electability. I think Hillary would have a harder time. Hillary's base is the Democrats who always vote. Bernie's vote is progressives and Independents (which generally need to be given reason to vote). She's spent the last month pissing progressives off and the right HATES her so much. Her supporters like to say that she's been attacked for 20 years and still standing, except her favorable numbers are pretty bad. Hillary Clinton will throw as much mud as Republicans, so facing her in the primary is a good way to get a taste of what Republicans will try (except that Hillary can attack from so many other angles than Republicans can). In the last month, Hillary's camp has tried:
--Sanders will take away your healthcare (Hillary herself, Chelsea herself, then Hillary again)
--Red-Baiting (Claire McCaskill, who effectively wrote Republican advertising. The RNCC used exactly what McCaskill said two weeks later)
-->As an aside, red-baiting would work a lot better if 1. Sanders was a Communist 2. The Republicans hadn't spent the last 8 years screaming about how Obama was a Socialist
--Sanders is racist (David Brock, who is worth discussing because yes, Hillary works with the man that smeared Anita Hill)
--Sanders has a gender issue (Madeleine Albright, Gloria Steinem)
--Whatever they can throw at the wall (Whenever Bill Clinton speaks. He's tried sexism, racism and a few other baffling things)
All of these have backfired badly, and have definitely helped Sanders more than anything else. Hillary's real strength is that she has more consolidated power and money than any other candidate on both sides of race. Considering how many institutional advantages she has, it is amazing that Sanders has as much chance as he does. If you asked me 6 months ago, I was for Sanders but thought he maybe had a 10-15% chance of winning the nomination. I would put that around 30-35% now.
I could write a litany on why Bernie over Hillary (I'll vote for her in the general though, barring her winning due to super delegates), but it basically boils down to this:
--She's shown no real commitment to fighting climate change, which is my number 1 issue. She spent her time as SOS pushing fracking onto countries that didn't want it and takes lots of money from the energy industry.
--She's devoted to a foreign policy strategy that has been failing the US for the last 60 years (since Iran in 1953). You would think that Iraq was a lesson, but it didn't stop Clinton from massive mistakes in Libya...Syria...Honduras...Colombia...Bulgaria.
--I believe that she's actually fully supportive of the TPP.
--I believe that she cares no more than Obama did to actually do something about Wall Street.
--She's either a horrible manager or completely incapable of learning (foreign policy implies the second). Internal emails showed that her campaign in 2008 was an absolute disaster. In 2008, she dipped into racism, sexism and bad overt attacks (If you don't know, Obama's camp believes that the birth certificate stuff started with Clinton). They all failed and basically guaranteed near the end that she wasn't going to win. Eight year later...she is trying literally the same exact tactics. As I noted above, she tried to make it seem like Bernie was going to take away healthcare and it backfired. Right after that, Chelsea tried the same thing and it backfired even worse. And yet...she just tried it again this week.
Any area where she has strengths, Bernie generally does too. I'm definitely not worried about "White Old Man" fatigue syndrome because Bernie will be filling a pretty major first as well: he'll be the first Jewish President. There's been a lot of indication that Bernie has gained a lot of support among Hispanics and Asians. I think he will keep Nevada competitive. He's now polling around 39% in aggregate averaged polls in SC, so 45% isn't too far off. Two months ago he was trailing Clinton by 40 in SC, now he's averaging 20.
On the R side, the only one that scares me at all is Kasich, and that's only because he knows how to hide how dangerous his plans are (He's Scott Walker who able to be nearly as toxic without drawing the heavy fire). The flip side is that Kasich is the one that will inspire the base the least.
A lot of Sanders policies aren't that far out because a lot of them are returning to what we had. I would note CK, that his policies tend to be very popular individually (as are most Democrat's policies). The problem Democrats has is packaging together cohesive strategies. If Democrats run to the right, they lose (Republicans beat fake-Republicans).