I have both T8 and T5. Had to take the T5 because I needed a new hood/light for my 66 gal and it was only available with T5 tubes.
At the shop they told me that T5 will give you more light/lumen for less Wattage, especially if using with reflectors. Not sure though if that means saving money cause the price of the T5 might make up for the possibly lower bill from the power supplier, and they still have to be exchanged after a while as well.
If I was into plants really or had a marine tank, I'd go for the T5, for keeping Oscars I don't see a real advantage. It's nice that they can be dimmed, but not sure now if the standard T8 can't:unsure: What I like about the T5 is their smaller diameter. Regarding brightness, I can't even notice a difference between the T5 and the T8 I had before, but maybe just needing glasses.