It's hard to tell. In terms of physical releases on a retail store shelf, WiiU has the Vita beat. Many retailers in the US don't appear to stock the Vita or it's games anymore.

In terms of releases through niche channels, Vita beats the WiiU.

12+ big first party games in 2017 for Wii U.

So, I feel there have been a few developments either way on this over E3.

On the WiiU side, you have NoA Vice-President of Sales saying:

https://mynintendonews.com/2016/06/18/nintendo-of-americas-scott-moffitt-says-you-should-grab-a-wii-u-while-you-can/

"When he was asked how many Wii U consoles were left, Moffitt responded by saying that “we will be transitioning production to NX, so our message to fans is to grab a Wii U while you can, because they won’t be around forever”."

Which isn't definitive but paints a pretty clear picture of a WiIU on its way out in 2017.

On the Vita side, a few things.  There were a number of games confirmed for physical releases in 2017 (Fate/Extella; Summon Night 6) but you're also getting things like:

http://forum.marvelous-usa.com/index.php?showtopic=28319&st=1500#entry923096

"Yeah, I quite like the Vita, and we've got two announced Vita titles that are receiving physical releases (Shantae and Cold Steel II), but... honestly, the days of Vita physical releases are numbered, and it's not for lack of desire to put them out -- it's because barely anybody carries them anymore! And in this business, we are absolutely dependent on retail stores: if we release a game physically and no retailer is willing to put it on their shelves in any decent quantities, then the game flops, plain and simple. And we publishers don't get to tell retailers what to carry -- rather, it's the retailers who decide what to order from us. All we can do is try to sell them on our titles being worth their time and money to stock."

Tom from XSEED pointing out that physical Vita in the west is really struggling.  This is compounded with the Atlus news that they're bringing across Caligula:

http://gematsu.com/2016/06/caligula-coming-west-via-psn-spring-2017

As a digital-only title.

If we're counting physical release, Vita should win by a landslide, purely because Limited Run Games seem to be going from strength to strength in terms of their Vita releases and they confirmed the signed a load more over E3.

If you're counting retail-only releases, I feel it's going to be a closer race.  Both will have LEGO titles, and Vita will have niche-releases which seem to be drying out while WiiU will have family friendly shovelware which I believe will also dry up, but may stick around at least for next year.