I'd mix it up a bit- 250' of one plant is just asking for pest and disease problems. Consider adding a some groupings of taller shrubs and ornamental grasses.

In my experience, creeping phlox has fewer weeds come up in it than juniper. Either one will take 2-4 years to cover, longer for the juniper.

Spacing depends on which plant and how much money you have. Phlox will go 3-4' wide in a few years, but for quick coverage you'd want to plant closer- 2'or less. Junipers would be spaced by the size of the cultivar you choose.

The link below will help you figure out how many plants you will need.

Here is a link that might be useful: plant estimating page

The juniper will take forever and is prone to spider mites, especially in dry, sunny spots. I tried to grow blue rug juniper on a sunny bank at the front of our property and the stuff grew so slow I swore it was dead. Sometimes the spider mites were so bad it looked dead. However, I have some pacific shore juniper in another area of our property that I absolutely loooove and it has never had any sort of pest or disease. It is one of the most care-free plants I have. *knocking on wood, crossing fingers*. Depends on the variety, I suppose. I'd do what a previous poster suggested, and use a mixture of plants in the area. There is no reason why you can't use 'some' juniper and 'some' phlox, among others.

Forsythia is great at controlling erosion, so are daylilies, do you ned to keep this a low to the ground kind of thing? Can you do sweeping patches of different plants, varying the height, texture and season of interest? Go to your local library and look at every gardening book you can find to find some pictures of sloped yards. You will see that the ones that look the best are the ones that turn the slope into an asset. 250 ft of even 2 things will not make you happy in the long run.

I appreaciate the info - thanks. As it turns out, the county convervation office is making the builder do more repairs than we thought. They have 'asked' him to make the slope less steep. We may be able to plant grass on some of the area, leaving only 1 steep section in the middle for groundcovering. I think I will heed the advice of multiple ground coverings.
How high would forsythia or daylilies get?

daylillies range from the short hybrids with 'fans'( the leaves) only a foot or so tall, to the local 'ditch' lilly, which can have 40" fan in full sun, and the flowers are right in my face in the summer (I'm 5'7")

the ditch lilies have the advantage of being cheap or free- and they spread freely in anything short of drought conditions. but they are the tallest.

the forsythia is a full shrub, can get 8' square without thinking about it, or can be pruned to shape. they can also be propagated by 'stapling' a branch to the ground, and letting it root.

in a full sun condition, there are a few other choices- spiderwort spreads like hateful gossip in the sun, forms dense clumps, and I'm in love with the deep violet-blue flowers...and if you planted it now, you could dig the clump up in september, divide it, spread it out, and have six or either clumps by next spring ( if you're in Bucks Co close, I can provide them for a fraction of what the nursery will want for them)

gout weed (I prefer the name 'false lamium' which sounds a little less painful) is another damned aggressive monster that's impossible to kill, and isn't unattractive- green and white striped foliage, spreads by seeds, runners, and roots, not too tall (the stems fall over when they get about 14" tall, and tend to root where they touch the ground)

groundcover roses is a lovely, but pricey option.

Rocks, since you're a local, can be a good option to interplant with the plants.

the upright sedums are also a viable choice for a slope around here- mine came from a single leaf I plucked from a roadside planting, it was a cauliflower-sized clump that fall, and it roots REALLY fast- you can break a 'branch' off and it will root in the dirt in a week with no trouble at all.