I think the best thing would be to wait until you've gotten your Wii U and played both games for a while. You'll likely have an idea of the DLC's value, as well as what the DLC is going to actually give you (since for example, HW's 'Adventure Maps' might not mean much to you right now)
The Mario Kart 8 DLC might depend a lot on whether you play multiplayer much (either online or with friends).
I personally don't play online much at all (I think I've spent 2 hours online since I bought it), but I still think it was worth the price they're charging for what they give you since they're really fun tracks. The Hyrule Circuit was especially a great track with a lot of really nice little details.
The characters may or may not interest you, that's a matter of preference though.
I have always used Yoshi since the SNES Mario Kart, so the extra characters were irrelevant to me since I was always unlikely to use them (though I do look forward to Isabelle)
Basically, if you're fine paying for Mario Kart 8 in the first place, you'll be happy enough to spend ?13 for another 4 cups. They could have easily charged more for what they're giving us here, especially since they're almost giving us half as much content than was in the main game itself! It's definitely a bargain.
Hyrule Warriors give you a lot of content, but it's largely stuff for 'Adventure Mode'. You're better off trying Adventure Mode for yourself first and seeing what you think of it. If you like Adventure Mode, then it will definitely be worth the price since there's a lot of content for ?14 (with more still coming).
I didn't really like Adventure Mode all that much (I must be one of few that doesn't) and as a result, I've not even touched 95% of the current DLC despite owning it.
It also gives you some characters, costumes and a few weapons but again, you're probably better looking at what they are for yourself to see if they even interest you. The costumes are largely 'palette swaps' and just a few skins at this point. The only character so far is 'Twilight Midna' (correct me if I'm wrong) and I think the only 'weapon' is Epona (who I personally found clunky and awkward to use)
You're really going to have to look at what each DLC gives you since it's a mixed bag and decide whether or not you think you'll even use the stuff you get or how much YOU think it's worth.
It is really worth the price for how much you're getting, but that doesn't mean much if you don't happen to like any of the content it gives you.
- - - Post Merge - - -
Another board where nobody likes anything.
But both DLCs are super worth it. MK8 adds enough and people vote for the tracks online often enough too, plenty of replay in that. No idea how people are saying it has no replay value at all *facepalm* That's like the one thing people say about Mario Kart in general especially thanks to the online mode.
Hyrule Warriors = most of it really worth it just for that 2nd map, it'll add a lot of gametime, but of course all the other goodies that come along with. If there are any plain costume packs I personally feel like I'd steer away from it just because I don't care for more costumes, like I don't want to just buy OoT clothes that do me no good but lookin pretty.
But...There's like...Nobody hating on anything at all. Just 2 people that don't play online with Mario Kart
