I recently did tracks through a RedDI and a U5 simultaneously. While laying the tracks, I liked the throaty tone of the RedDI--and didn't like the unforgiving resolution of the U5...but, as the arrangment gets closer to mix time, the U5 works better for the sound I need. You can soften it a little with a La2a
(or similar)...but, there's no "make it deeper and sit better with the instruments" device for the RedDI.
In the end, it's nuance--both are great DIs. They're just two very different sounds...and at least for me, the one that sounded coolest alone wasn't the one that sounded the best in the mix.
If price were no object, I wouldn't limit myself to one bass DI.
The U5 is stellar at clean and deep. The RedDI is king of the tubey throaty mid bass thing. The Great River Me1nv was king of the sort of solid, deep, 2d, wall'o'bass sound. I wouldn't say that of the three there's a "better/worse"...just three very different vibes. So, I'd say any of the htree are "the best"--for whatever sound you may be looking for.