A third choice is to sow and put the pots outdoors now, either in a cold frame or someplace they are protected from foraging birds, rodents....or you can protect by putting a piece of screening or chicken wire over the pots. Fall sowing lets mother nature take care of the temperatures and at a natural progression - I've sown seeds for several things this month and have the pots in tiered carts on my deck. I top seed pots with a layer of grit or very fine aquarium gravel for those things that will take several months to germinate - discourages moss, algae. You could sow and leave indoors if you are sterilizing all your materials - sowing in potting soil indoors in an enclosed space can lead to some interesting fungal things growing on the soil surface when you are keeping them moist, and, you lose what could be some benefit of fluctuating day/night temperatures in conditioning your seeds.

Gray Dogwoods sucker so much that they end up growing in a dense thicket. I think you could plant them 2 feet apart and be OK. In my fields the gray dogwoods stems are about 6 inches apart. I'd expect Elderberry to grow taller that Gray Dogwood and not sucker or spread as much or as rapidly. I would give the Elderberry some separation from the dogwood - don't plant a dogwood within three or four feet on either side of an Elderberry. One drawback to Gray Dogwood as a screen is that they are deciduous - in the winter they won't provide much screening. Same goes for Elderberry, which are even less effective as a winter screen because they have far fewer stems. I think you will see lots of birds eating the berries of both shrubs. Both shrubs will fruit much better in full sun than in shade. I have Gray Dogwood growing throughout the woods, but see little fruit on the shade-growing dogwoods. Same goes for Elderberry. Both need about half sun to grow much fruit. If you have lots of shade you should consider a different shrub. Mapleleaf viburnum (Viburnum acerfolium) would be a good choice, although it doesn't grow upright like Gray Dowood - it tends to sprawl.

Grey dogwood never gets tired of sending up suckers, the suckers eventually get as big as the plant. So unless you want a gazillion plants in one spot, you have to remove them. What you do it dig down about 6 inches or so, a few inches away from the sucker. You'll find a very strong horizontal root running in a straight line--the main plant sends up new suckers along that line. It's easy to see, because the suckers are often lined up in a row. If you want to slow the plant down, dig up that whole horizontal root. Each sucker will be easier to establish if it has a part of that root along with its own root system. The sucker's own root system will be rather small. Plant as for any bare-root tree or shrub--dig a hole or slit just big enough for the roots, don't amend the soil, firm the soil very well around the roots, water well, and keep the area well watered for the entire first growing season. If you're going to dig and replant these, do it before the weather gets too hot.

You can take a photo now and it will likely be IDed even without leaves. It could be either of the two small trees suggested above based on location.