I have always used double-quotes and I think we'd find that 95% of links to aswell.
1) I suppose but it is practically the same since you insert a double-quote with two different hands (I.E. It's still pratically one keystroke)
2) Well, if we want to get pedantic using single-quotes to escape PHP strings is faster for PHP to interpret and because double quotes are wider they are easier to see in the string.
That's my view anyway.
As for quoting attributes both single and double are valid, with similar quoting rules (Singles ok in double, double ok in single)