With the rumours we have to go on, nearer to PS3 than PS4. And my main reason for thinking that is that if the hardware was a significant advance, it'd be showing in the launch titles. Every generation that has an order of magnitude improvement in power gets a notable improvement over last-gen graphics, even if it's just an increase in resolution and texture detail. Wii U's launch titles are around current-gen levels, which means there isn't a massive amount of power beyond that - not unless the Wii U developers are struggling to get more than 10% utilisation from the hardware! But we don't need to go by that logic. We have dev comments telling us Wii U's performance is around current-gen, and a small case. All the evidence points that way.
The only way Wii U could be closer to PS4 than PS3 is if PS4 isn't much of an advance at all, but the rumours are saying Sony are going fairly powerhouse again, although not monster power like their previous consoles so the visual advance may not be as pronounced as we are used to between generations.