The 5600 has only 32Mb or RAM, the 5500 has 64Mb.
If your choice is between the two, I'd go for the 5500.
You'll also find that support for the 5600 in terms of the number and polishedness of ROMs is no where as good as for the 5500 (as fewer devs ever bought 5600s).
If you can, I'd stretch to a clamshell model (anything but a C700 which again only has 32Mb or RAM).
If you wonder why I'm going on about the memory, do a search of the forum for 'swap' and see the troubles people have when there isn't enough.
Just my opinion of course,
Si