I guess the question becomes, what does one mean by "better" for emulation needs. Undeniably, the Wii U can much more faithfully recreate the 2 screen experience of the DS than the Switch. And thanks to DS VC titles and injects, we can play 100s of DS games on the Wii U flawlessly. Unfortunately, not every DS game can work on Wii U with injects (gen 5 pokemon games and Chinatown Wars are the big examples that I recall), but the vast majority of them work great. Now I know OP was asking years ago for a proper DS emulator and not just VC titles and injects, but if they do opt for VC and injects, the Wii U is nearly perfect for handling most DS games. Of course the Switch can probably emulate nearly every DS game without issue, but you lose the proper 2 screen experience in the process. So I guess it becomes about preference. If you ask me, even though the Wii U is less powerful, I would argue that it is the better of the 2 devices to emulate DS games on, even if some games don't work.