imo the 180 is key just 5 inches shorter i had a oceanic 215 that was 29 high and was unable to do nice rock work feed coral on the sand bed with out taking a bath and also with a 30 high tank will need 400w mh to hit the sand bed i have yet to find a fixture that is t5 that will work that is under 1500 again jmo
180... Bigger isn't always better! I have a 180 and it is deep enough. I couldn't deal with the extra height. Only way i would consider it is if you are considering a pretty deep sandbed. Otherwise is 30 gallons really worth the extra hastle... I would rather go wider than higher!
with a tank that big and heavy have you put any consideration into where you are gonna put it? When I first bought mine I didn't realize how much the thing would weigh. Luckily I have it on basement slab!