I have the 128GB Sandisk Ultra Dual Drive and have just done a test with a folder (4.05 GB, 434 files).
- Writing to the stick via USB 3.0 (type A): 3:14 minutes (approx. 21MB/sec write).
- Reading from the stick via USB 3.1 (type C): 0:29 minutes (approx. 140MB/sec read)
- Writing to the stick via USB 3.1 (type C): 2:30 minutes (approx. 27MB/sec write)

Which is it, 25 or 50 MB/s or why the difference? No wonder, the speed is not even in the small print on the manufacturer's website.
But they advertise up to 150 MB/s with USB 3(.1). Wow! My old USB 3.0 stick writes just as fast, if not faster. Not to mention reading.