puddingstones -
How great you're planting a stand of these, as they are beautiful. I love seeing groups of these together and it's a shame that one of our local drugstores does not take care of its stand of these shrubs, as they are wonderful at many times of the year and thus provide interest more than once!
I must say, I'm not a fan of the yellow variegation in C.a. 'Spaethii', as I think it's "brightness" seems to disappear. C. S. 'Bloodtwig' may be too large for you, growing relatively slowly but reaching a floppy 6-15' tall as wide, so if you choose this one, space them accordingly. I think the most plain of the bunch is your last selection, the C. s. 'Red Osier'. Sometime branches stay green, flowers and berries are pretty dull-cream, and foliage is plain. Incidentally, they call it "Osier", as that means "pliable twig", and young branches can be used to weave baskets and such...just learned that, so thank you!
In closing, my personal fav in the red-twig color (as I absolutely love the yellow-twig cv. called Cornus alba 'Bud's Yellow', which I saw at Weston Nurseries in Weston, MA)is the Cornus alba ÂElegantissimaÂ.
I've included a few notes. You may have already considered many of these, but nevertheless.
A perfect time to plant these beauties in the Fall, too!

Info re: Cornus alba sp.
1. It is generally a shrub that exhibits low allergic reactions due to air-borne allergies in people.
2. The species is synonymous with C. alba, C. sericea, and C. stoloifera.
3. They bloom in late May and in fall have lovely leaf color, and of course, deciduous in winter, showing off that lovely red-twig color.
4. ItÂs really considered a "Low-maintenance" shrub
5. Can take being planted in clay soil that is neutral to slightly acid.
6. Great for a planting in wet sites.
7. It would look lovely to have my fav, C. a. ÂElegantissima as a backdrop to some lovely cultivated daylilies, as their creamy-white variegated leaves would really show the plants off.
8. They are also great for area with exposure to cold.
9. They can be sensitive to salt spray, but have a pretty good tolerance to it.

10. The species has its origin in Siberia.
11. It's a perfect spot if you can have them in full sun, in winter, to show off that bright red and/or crimson suckering bark.
12. And if you need more, softwood cuttings can easily be made in the summer.
    Best,
    PP