I do not understand your need for side light, is it because you have had lights that did not light up much to the side and you want something better? Or, is it because you want traffic to see you from the side?
I do not know if the Luxos U and B have the same light pattern. I think it safe to assume that the Luxos U and B have the same distant LED and reflector. But at slower speeds the U has some different LEDs that light up more close area. That power is shifted to more distant focusing LEDs at higher speed. I have no idea how it determines speed, I assume AC frequency. It is unclear from the B&M website if the B has those close area LEDs. The AXA Luxx 70 Plus also has some additional close in lights at slower speed, but that light has a pretty narrow pattern and I would not recommend it if you want good side light.
The B&M website is of no help on the B, this page (english version) describes the X for detailed text.
https://www.bumm.de/en/products/dyna...dukt/179b.html
I like my Luxos U and like the light pattern it has. Due to the documented problems with water ingress, I keep a sandwich sized plastic bag in my handlebar bag and I would be hesitant to use that light for commutting where the bike gets parked outside every day for hours.
The Luxos U example pattern on their website shows the 90 lux pattern, that pattern is probably not sustainable for a long time because it relies on battery power for the additional 20 lux over the standard 70 lux. More on that light here:
https://www.bumm.de/en/products/dyna...dukt/179u.html
That battery is charged by the dynohub.
Luxos U 90 lux pattern below, this is from the B&M website, but I do not know if that is the pattern for slow speed or higher speed:
The IQ-XS is another light that I have,
https://www.bumm.de/en/products/dyna...di-silber.html
I like the Luxos U pattern better but the IQ-XS pattern is almost as good in my opinion. Their light pattern is below:
If however your desire for side light is for traffic to see you better, I think reflective sidewalls on your tires is a better option, then you are using the light from the car which is much brighter than your low wattage bike light. But you want to make sure your reflective material stays clean, not muddy.
The IQ-XS is fairly new, thus there is not a lot of information from users on water ingress susceptibility. I recently bought a second IQ-XS for another bike.
Regarding water ingress, all B&M headlights that I have seen are not well sealed at the back and the bottom of the light, thus you need to mount it where you would not get tire spray in wet weather.