the choice is yours really. ours has concrete foundations with soil beds with paving slabs done the middle.

Our bulb house has slabbed base on one side with staging and on the other a deep sand plunge... It really depends what you plan on using for.

Bear in mind that if you want to grow tomatoes in a soil bed its recommended that you change the soil every so often. I was happy with soil in my last one but would probably go for a concrete base another time. An advantage of soil is that the roots can go outside the house and so watering isn't quite as critical.