hello

they are two different species. Quite similar in growth, the C bipinnatus has white to pink flowers, C sulphureus orange-ish ones. Of C bipinnatus there are lots of cultivars, double, picotee or with rolled petals. And they vary in height,older cultivars grow easily 4-5 feet, newer breeds tend to be more compact.
So it comes down to your taste for colors. Last year I grew C sulphureus in a heavy clay soil. (We are still working on he soil, and I didnt water them very much), and they grew about 3 feet high, with lots of smallish flowers. I think they would do better in a container or with proper feeding.

And I would sow them right on the spot. Both are great, for cutflowers too.

Well, have a nice gardening season 2010,
cheers, Lin

It may not apply in your climate, tropicsnut, but I have found C. sulphureus to be slightly more tolerant of heat and clay soil. Botanical Interests used to offer a yellow form of C. bipinnatus, but I have never actually seen it growing or blooming.

Kevin : )

Thank you for your replies. :)

Started with sulphureus after seeing them in the neighbourhood. The ones in the ground flowered but the ones in pots didn't. Possibly because of the fertilizer i added before reading up on the plant. Will try bipinnatus now. The pictures on the net looked gorgeous.