Im going to need a retaining wall. 5ft tall by 35ft long with a return dying into a hillside. I know that there are all manor of block walls with different indexing systems and of course poured concrete. It does not have to be vertical. I am going to be reaching out for bids but beforehand Id like to get a bit of knowledge. Costs of block vs concrete? Best block systems? Do block walls 5ft tall properly done actually hold up long term?