If you have the money and can afford it, spend it. I have both because specific times require specific equipment to do the best job. I used a disk for years and ended up buying a tiller because there were times it did better. Same goes for the disk. I plant for numerous clients every year and both come in handy. I don't mow fields before I plant and a disk works best because it doesn't clog up. After the field is down and the grass dead, a tiller lays out a flat and firm seed bed. Each situation is different, but I like to have the right equipment to do the best job possible in the quickest amount of time.