I doubt either will provide much of a windbreak for you in winter. True pampas (Cortaderia sp.) is unlikely to be hardy in zone 5, let alone a 4/5. Sporobolus wrightii only reaches about 3' in leaf with flowerheads sometime achieving 7', but it will be deciduous for you and is unlikely to stand up to much wind in winter. I'd look to some other type of plant - conifers, perhaps? - to provide an effective windbreak.

I agree with gardengal, as usual. An ornamental grass is not going to provide much of a windbreak for you. Her suggestion about conifers should be considered.

However, if you do want to try an ornamental grass, perhaps consider Panicum 'Northwind'. It is very strongly upright at about 6 ft., dense and fast growing. It seems to collect the most snow for me here. It is deciduous, however, and will deteriorate as the winter goes on, so not much of a windbreak.

If you just want a large ornamental grass try the Huron series, they would be hardy for you.

If you have significant snow cover all winter you can probably do OK with the Z5 grasses.

Polly

I am in SW Colorado, z5 and have Dwarf Pampas grass growing in a warm microclimate on the south side of my patio. It's in clay soil, but it has roadbase on top of it (gravel/soil mix, part of the driveway) which I amended with lots of organic stuff at planting time. It gets about 5' tall, with plumes to 6-7'. I am going to try Giant Sacaton in other parts of my yard. Doubt it can cut the wind here, but I am more interested in blocking an objectionable view (neighbor's trash pile).

Happy Trails,
Michelle

This is what I love about this forum! Sporobolus wrightii is a gorgeous grass, and can apparently be grown from seed. The seed hunt is on!!

In case anyone's interested, I found seeds for Sporobolus wrightii (and other neat stuff as well) at Plants of the Southwest.

Here is a link that might be useful: Plants of the Southwest