IMO it is worth it regardless of what you want to keep. Wide tanks are awesome, I love the extra dimensionality of watching your fish swim towards/away from you rather than just left-to-right.
Unless that $300 is going to absolutely break the bank then go for it, six months down the road I guarantee you're not going to be sitting in front of your tank thinking "gee I wish I hadn't spent that money on going wider".
I have a chance to get a giant gourami but don't really want to until I have another tank. It's currently 10+ inches and if it's a prick I have no other Tank for it and I don't want it to harm my current stock.