With the release of the Island of Blood-set this question got even more relevant, as the shields are stuck on the models in the pack.
I started thinking, as it would be highly appreciated if I could use models from IoB to model my slaves, which will be my first unit to the army. ( Test subjects, as appropriate )
I chrunched some numbers :
Slaves whith shields relative to w/o
Against strength 3 or less ( i.e. no armour mod )
+ 15% wounds/point
+ 30% better combat resolution
Against strength 4 or more ( i.e. no 6+ save )
- 4% wounds/point
+ 15% better combat resolution
Alas the shields seems to be the more point-effective solution att first glance. However, you may be more likely to throw the slaves against deadly foes to
tie them up, they would definitiley be s4 or more. The choice really comes down to preference, though I surely can invest a 4% point-waste on these guys to get better combat resolution and greatly increase the effectiveness against s3. I mean if that point investment tips one combat every one and then, it will be worth it.
In the end I'll go for the shield option as my choice of slave models will increase dramatically without sacrificing efficiency.
I've already started sketching for the base-layout for the slave unit.
For the record, I'm also going with 35-rat strong units of slaves. From experience I feel it would be a good compromise of size and points for their duty. I've met some really large slave units (50man) and some really small (20man) and were not impressed.
Over and out!