I much prefer 5 * 1024 * 1024. I have to stop and do some reasoning with 5 << 20.

Correct, to me it's idiomatic. I realize it's not for everyone, so the ultimate best code is probably the way it was written ... On the other hand, sometimes it's fun to push the envelope just a tiny bit, and hope that readers will actually learn something from the code. But that's a thin edge to be balancing on.