We are getting ready to build and are getting estimates from builders. For basement walls, all things being equal, which is better: poured concrete or reinforced block walls? (Reinforced block was described to us as block walls with concrete with rebar poured into the block openings.) We were told that it is easier to get straight walls with block. Not sure who to believe . . .

Just my $.02 worth, but I'd be leary of block walls for the following reasons:

I grew up in a house with block walls for the basement. They always leaked, due to the porous nature of block.
If block is installed properly, the holes in the middle of the block won't line up to properly fill and insert rebar.
Mortar joints on block are easily compromised.
A 6" poured wall is much stronger than 8" block walls.

I've been doing extensive research on foundation walls recently and suggest you look into 2 more alternatives as possibilities for yourself:
ICF's (Logix or Quadlock)
precast concrete walls (like Superior Walls)

Block walls would be my last choice.

My experience with concrete walls has been all bad, and with block walls, all good.

Reasons:

Poured-in-place concrete walls are NEVER square, because even if the forms are set up carefully (which is seldom the case), when the wet heavy concrete is poured into them, they shift. Poured concrete walls are also VERY difficult to modify in the event of an addition or alteration.

Block walls are easy to get square and level, because a careful mason can bring them back to line if they begin to drift. You don't say what region you live in, so I don't know whether you need reinforced walls for seismic reasons, but most foundations in the northeast region are unreinforced block, with only light masonry reinforcing in every second block course. If they are built and parged properly, I think they look best also. And if you need to modify the foundation, it is relatively easy to do.

A lot also depends on what is customary in your region. In one area where I practice, you almost never see a concrete foundation, and my favorite builder there refuses to build them. In the other area, you see more concrete, but the majority of homes still have block foundations. However, if you go just a couple of dozen miles further east, to Long Island, New York, you rarely see block foundations, and concrete is almost universal. It is a matter of what the workmen are accustomed to doing.

As far as moisture and water are concerned, if each is done properly, there is no difference. If each is done badly, it doesn't matter, there will be problems with either. Your architect should pay careful attention to the soil and ground water conditions when the excavation is done, and make recommendations after he sees what the conditions are. Waterproofing (which is different from dampproofing) the wall and proper backfill are cheap when done initially, but retrofits can be more costly.