Seriously, take it. You aren't as important to your current employer as you think you are.
Oh, they will cajole and complain. But you will kick yourself for the years to come because you will wonder what would've happened if you had taken the job.
And, if you say, the management are really your friends, they'll understand eventually. Good management understands the value of keeping even leaving people happy - because after all, the employees might want to come back some day.