It depends on if you have any fish that are outgrowing your tanks, what they may be, what you want, how long you're willing to wait. What I'd do is get the 75 for the $100 for now and start saving up again for a 180 or whatever size you find for a good deal. I wouldn't be picky with what you want. Like don't be "I want a 180, not a 179, not a 181, it HAS to be exactly what I want." Be open to different size tanks. If a great deal pops up for a bigger tank or maybe a smaller tank don't be afraid to jump on it.