I think it would be a nice upgrade if you're looking for something much deeper. I have a 150 whcih is 18" deep, but always wanted at least 24" deep. I just don't have the room in my house as my dining room is already a bit cramped with an 18" deep tank, so 24" or deeper would not let me fit my table with people sitting. Deeper will give much more opertunity for interesting aquascape, but will likely need more lighting to reach the front and back.
Going from 155 to a 220 is an upgrade, but it's up to you if you think that you'd be happy after a few months with the upgrade, or wish you would have waited to go bigger. Every time I've upgraded my tank I always doubled in size or more. I think if I went from my 150 to a 220 I'd be happy for a few months, then may start wish I had waited to get a 300. All depends on what makes you happy. Funny how big tanks start to look normal sized after you look at them for a while.
Not sure the difference from 5 to 6 ft for tangs.