After their successors release what do you think the positive legacy of both will be?

3ds: The handheld that stood up against the mobile invasion
WiiU: Pretty awesome gamez

Essentially that. 

Most consoles usually leave a good legacy, despite sales and such so I'm sure they'll do well in the record books.

The Wii U isn't anywhere near the failure of the Visual Boy..

Both will be known for their great lineup of Nintendo-published games and in 3DS' case a bunch of solid 3rd party Japanese games.

3DS should have a great legacy at this point. As others said, it is the handheld that withstood the mobile takeover and still managed to finish with lifetime sales that will likely only be 15m or so below the 80m selling GBA. It also will go down as a total JRPG haven where the genre flourished like it hadn't in years, and a strong software lineup in general.

Wii U should have a similar legacy to GCN. Great software lineup that not nearly enough people bought the system for. But like GCN it won't be an outright failure, just a disappointment.

Depends. If the new Zelda gets pushed to NX, then Wii U will be the only Nintendo console to not get a Zelda. In that case, the Wii U (at least in my mind) will go down as the worst Nintendo console ever.

The 3DS will be remembered as one of the greatest handhelds that survived the rise of mobile with a fantastic library.

The Wii U will be much like the GCN and DC in that it isn't experienced by many but it's so quirky in its structure and games that it will have a sort of following. (I can see a lot of the Wii U's games becoming very desirable to collectors in the future)

Positive.

3DS- best console of all time. okay that's my opinion but still. AND a large chunk of greatest games of the gen are there.

wii U - gems and amazing exclusives. Next GCN but sell worse yet better games than GCN.

WiiU is the new Dreamcast.

But with 4 - 5 years of live instead of 2.

So pretty f*cking awesome machine for me :P