Hifi boys like silver content, but a lot of the silver solder around today is lead free, the silver is there to get a bit more flow.
From July 1st you can't really use lead in electronics manufacture, and under the list is electric guitars, so for the EU ROHS laws all manufacturers must switch to lead free solder, trouble is it just doesn't flow like the good old lead solder!!
Most important when you choose a solder is get one that's relatively pure, old tandy solder used to be full of impurities, Rapid Electronics do a great high grade solder, much better than the generic multicore solder most places sell.
Wonder solder is the best low lead solder around, Russ Andrews and some other high end hifi boys use it, it's dear though and I doubt it'd make a difference in a guitar, you're better off just getting a really good solder joint that has mechanical strength.