You want a lot of space and a simple, clean, uncluttered rom? Go for Harmonia.
You want extra space, lots of customizable options, and nifty bells & whistles? Go for BACKside.
No one wants to choose for you, of course, but that's where I would start.
Also, last I checked Harmonia was based on Froyo, while BACKside is based on Gingerbread.