I suppose you don't have any personal preferences concerning the background of the two warbands (as otherwise you probably wouldn't be having a hard time deciding)? How about the models? Maybe you prefer the marauder models over zombies and skeletons? The Marauders have a lot of hobbying potential as you can give them mutations and have them follow a certain Chaos god. The Condemned is always a challenging conversion. The Restless
Dead have just as much conversion potential if you decide to do some scarecrows or even a Bone Goliath (see Hired Swords section of the BTB book). Are you an Undead fan? With the zombies and necromancers you can easily use the models for two warbands: the Restless
Dead and the Undead from the core book as well.
If the hobby aspect is not a deciding factor, how about the game style? Both warbands are focused on close combat, but the Marauders are much faster (more so if you play a mounted Hung tribe). The RD are slower and have two spell-casters. They play very differently. The zombie bomb skill is pretty fun and scares the hell out of your enemies.
Personally, I'd go with whatever background/fluff and minis I prefer because having enough motivation and enthusiasm to finish a whole warband is really important.
No matter for which of them you decide eventually, note that you can submit them for the Show Us Your BTB competition