The HD4670 is more powerfull,5550 quite a bit weaker but has DX11,imo though the HD5550 is too weak of a card to take advantage of DX11 anyway...personally id go with the 4670.
id get the hd4670, i just got one havent used it yet but it is very good for the price and can play through games with good details with good framerates ( at least from what i've read i got this which is 54.99 after rebate
DirectX 10 and 11 are similar, but not equal. There are very few differences. Windows XP supports up to DX9, Windows Vista and Windows 7 support up to DX11. If you care to read more, see Wikipedia: http://en.wikipedia.org/wiki/DirectX#DirectX_10