GW & Mantic (right) Zombie
The figures are about the same height but the GW model is bulkier. Both are plastic kits but the GW is multipose with all sorts of options. The GW is disappointingly sculpted. It has very exaggerated cartoon features with enormous hands and the bits don't fit together too happily.
GW Zombie Regiment
On the plus side one can put together a really chaotic looking regiment with all the GW options but the models seem very crowded on the movement tray because of their bulk.
Mantic Zombie Regiment
The Mantic models are much, much faster to put together and are easy and quick to paint. Speed is not an entirely irrelevant issue when creating zombie hordes. On balance I think they look as good as the GW models, albeit with a very different style.
In terms of cost, the RRP of the GW set is £20.50 for twenty which is a fair price considering all the options available. The Mantic zombies currently weigh in at an RRP of £24.99 for forty or £29.99 if including the metal upgrade parts for the command models. Command models are purely decorative in Kings of War and command zombies strike me as a non sequitur so I don't think they are worth the extra fiver. You may feel differently.
So there we have it.
Both sets are decent value for money but on balance I recommend the Mantic offering because : (i) they are cheap - two regiments for £25, (ii) they are easy to model, and (iii) I like the look of them.