Nice to see someone else being considerate of their fishes' space requirements.
What do you have in the 30" that would do better in the 36"?
I only ask, because volume wise, it makes a difference, but as for swimming space, it's really not that much more.
Don't take this as me being negative, or telling you what to do.
Personally I'd review the fish, and tank requirements and then I'd just watch them swim in the tank. If they seem cramped, upgrade away!
Just beware, eventually MTS will get you, and you'll be upgrading to a 250 gallon in the wall.