Comparing Skeletons:
Now that built both Mantic and GW Skeletons I think I can make a bit of a comparison here, from my point of view:
Mantic Skeletons are much quicker to assemble. Like, a quarter of the time.
GW Skellies have more options (there are more heads, spears, shields, and swords included than there are Skeletons) so you can customize them more
The GW ones wear less armor/clothes, which I aesthetically prefer. The rags they wear have needlessly complex shapes though. They do look OK but they take longer to paint than they look. I like the armor better on the Mantic ones and it is easier to paint. I just wish they had a bit less of it.
Mantic ones are realistic scale, while GW ones are heroic scale. Both have their advantages. The Mantic ones have such small details that you can hardly paint some of them, at least not at my painting skill level. The details on GW Skellies are bigger. Both have lots of details so you can paint both kinds pretty detailed if you should choose to do so. Heroic scale makes the weapons on the GW ones look more interesting though, I give them that.
The art style is more consistent on the Mantic ones, they look like soldiers from the same army. The GW Skellies have a weird mix of Viking style shields and other ones for example, some don't fit well to each other. Same for the helmets. It makes them look mire diverse though. So that's both good and bad.
The GW Skelton banner looks cooler, but it is kinda weird if think about it: why should a Skeleton army have a Skeleton on the banner? Did they make it after they were dead? The Mantic ones look they were dead, resurrected, and then just took what they had and marched on. The GW ones look more...engineered to look cool? Which to me is kinda strange.
The Horn blowers of both armies look cool, and in both armies you can decide to build them or not, there are enough optional pieces in the kits for that. It is a bit easier for the GW ones though.
Mantic Skeletons contain an undead dog and two skeletons coming out of the ground, that you can use either as troops or as objective markers, which is very cool.
GW does leave you with some spare parts as well. Lots of them actually. There are 24 heads included for 10 Skeletons for example.
GW Skellies are more than double the price compared to the Mantic ones. Both have square bases included so you will need to buy round ones. I assume that will change with the upcoming repackaging if GW Skeletons though.
All in all I'll say I like both. If you want to play in GW stores or on GW tournaments you will have to choose GW ones, and they do look a tad better and more customizable.
However if you just want to build 120 of them for games with your friends you will save 70€ or so and a LOT of time if you choose Mantic.
I will post more thoughts and pictures of both of them once I have finished a decent amount of both.