I've been working in the same lab for my entire time as an undergraduate. I'm close with the PI, and he has connections, so he's definitely going to write me a letter. The problem is, for my other letters, I'm not sure who to ask. I have a few profs who I've done well in classes with who I think could write me good letters, but we don't know each other in the same way as you would someone who you've written papers with. On the other hand, I've been working with one of the postdocs in my lab pretty closely, and we've worked on two papers together. I don't think he has many connections in the US, though. Is it worth asking him for the personal letter? Or should I ask my better-connected professors, who are more likely to know people at these universities?