They all have good and bad points. I tried to just stick with one or two and ended up buying them all! (Have spent too much on this kit! IMHO:
Modelbrass - good, mostly just for the under flood vents, tho some folks love the metal deck
Eduard - excellent quality, has fore and aft metal deck bits and numerous details
White Ensign - orig kit is excellent, many wide variety of details, and the extra "U-Brass" kit is also excellent, tho I am biased. It has replacement torpedo doors and surrounding flood vents with variations available, templates and such for all floods, shackles for rigging and more. Available at WEM, but check out more info at:
http://www.rokket.biz/models/modelsweb/rokket/u557/ubrass.shtml
Yankee P-Hull - I found it dissapointing: bad fit, bad 3 part match up, innaccurate, wrong radius, waaaay too expensive, poorly made. But that's me.
Save up becasue it's all going to be tempting!
AMP - Accurate Model Parts
Fabric Flags, AM Uboat Goodies & More
http://amp.rokket.biz/