I don't want to pay an astronomical number for either and I need something that is good on gas but is also sporty and fast. I also need room to put some subwoofers and an amp in it,I don't want to spend more than 18,000.

Buying a Celica would cost you a plenty of pennies compared to a Scion which is cheaper,less fuel consumption,quite powerful,more spacious and although it's not very sporty,you can do a simple yet very attractive looking sports mod if you refer it to a right mod shop.

Toyota is always the best!

The cars are both good and offers a lot more accessories and goodies. It all depends on the buyer according to their budget to choose for the best. Either a Celica or a Scion can give you a good on gas option depending on your choice of engine performance. Every time you add a performance part, their is a corresponding consumption, so the least performing small horsepower engine is best for good on gas option.