Think about it, concrete is solid and has no seams/joints as it is poured all at the same time. Block, ok so you have a seam/crack every 10" in height and one every 18" in length. So concrete MIGHT crack some day, block will have cracks from day one. Go with concrete, tar the outside and use Dry-Loc on the inside and you will never be sorry. Go with block and you will one day wish you had gone with poured concrete. All of this is based on my contracting experience, homeowner experience and what I have seen over the years. So all in all this is JMO. Good luck!