Both are beautiful and stunning in their own right when full grown and in bloom. Companions can either play out opposite colors or be similar, you can go for similar silhouettes or you can go for some contrast.
If you want a varied texture and heights but colors that stay similar you could go for some purple/pink color like purple cone flowers (echinacea)staying with pinker, not orange-y, varieties and going for one taller and one dwarf size. This site has some good choices to look at.
Purple Coneflower* - Pink Coneflower The blues and purples combined with the pinks will be a restful but pretty look.
You can try similar textures by using medium sized ornamental grasses. A blue fescue would fit as a similar look, or for some contrast try something with pink color like Hameln (aka Dwarf Fountain Grass)
http://www.customgardendesigns.com/g...thers_so_2.jpg or Miscanthus sinensis
http://www.customgardendesigns.com/g...te_aso_5-8.jpg
A really pretty combination and easy care would be any white daisy. There are several shorter Shasta Daisies that would be really pretty all around the larger plants. One shasta starts out pale yellow and turns more white (it is interesting to watch mine change color as it goes from bud to flower) it is called Broadway Lights. There are also white coneflowers, a good one would be Meringue:
Echinacea Meringue - Coneflower
If you want to play up contrasts go with flowers that are yellow like coreopsis, which come in several heights and leaf types. A short one that just seems to thrive on neglect would be a "mouse ear" ("Nana is the first named version I can think of) and there are a host of others as you can see here:
Plant Picker A really durable plant that now has a completely yellow version (I have several now) is Gaillardia, the variety would be:Mesa Yellow
All-America Selections
Some people love the contrast of orange to purples and blues. It is not my cup of tea but you could use other Gaillardias that are the more traditional orange and yellow to get the effect. I have and love a dwarf variety called 'Arizona Sun'
Gaillardia Arizona Sun -- Bluestone Perennials, Inc but there are also taller ones.