Concrete blocks are often preferred by architects and engineers over poured concrete. They have greater compression strength than poured walls and there's less likelihood of variation due to plant and contractor errors/short-cuts.

On the other hand, they do have less lateral strength than poured concrete and skilled blocklayers are needed. In our booming market, where speed is necessary and labour is dear, poured is the overwhelming choice.

Is 40-50 feet accessible for a pour? Sure, with a pump truck.

Concrete blocks are often preferred by architects and engineers over poured concrete. They have greater compression strength than poured walls and there's less likelihood of variation due to plant and contractor errors/short-cuts.

On the other hand, they do have less lateral strength than poured concrete and skilled blocklayers are needed. In our booming market, where speed is necessary and labour is dear, poured is the overwhelming choice.

Is 40-50 feet accessible for a pour? Sure, with a pump truck.

The pump trucks work fantastic but it would probably double your cost....LOL

Depends on what you intend to use the basement space for.
Every seam between every block is a potential leak.
Does your contract specify what type of foundation?

but it would probably double your cost....LOL

I don't know where you're building. But at a typical Canada/US $500-$750 minimum charge for four hours (includes travel time) vs. the extra labour for wheelbarrow work, it's usually a bargain.

Every seam between every block is a potential leak.

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Dimple Membrane Waterproofing Photo: Mason Contractors Association of America

As is every settlement crack in a poured wall. In any case, a dimple waterproofing membrane, standard practice in some areas, equalizes the leak potential between the two types of walls.

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The photo in my first post (sorry about the double!) is of a small boom truck. You can easily get trucks with up to 140 feet of horizontal reach.

What are the soil conditions like? In expansive clay soils, blocks are a no no. I hired a pump truck for about 12 yards of concrete and it cost $500.

In expansive clay soils, blocks are a no no

Expansive clay can cause havoc to any type of foundation. That's why foundations/slabs/piers and the type of backfill used should be designed or reviewed by experts in your locality.

Here is a link that might be useful: Expansive Clay and Foundations

The contract says poured, but the blueprints say poured\cmu block. I know I can stick the contract to them and force a poured foundation, but they've given me a few breaks on other things so I don't want to resort to that.

The blocks will have some rebar and the cells that have rebar and the cells that are under a primary load bearing member will be filled. The outside will be parged with concrete.

Their reasoning is this is how my existing foundation is constructed so the new walls be no worse than what I have now. My house is 30 years old and it does't make sense to make the new foundation last a lot longer than the current one.

My soil is mixture of clay and silt.

My house is at the top of a hill and I've never had leakage in my basement.

The guy across the street from me has his own concrete business and says poured is better, but i'll be OK either way.