Both of these items is only usefull when they are needed for a specific reason. UV is for killing stuff. Firstly algae, and after that bacteria and viruses. Most of us do not need to use a UV. That said I do have one. I needed it because I was getting imported fish from water with a pH of about 4.0 and TDS under 20 ppm. Neither of these parameters is very common in the wild or in tanks. I also have an RO/DI unit for these fish. I needed to eradicate potential diseases in the tank before the wild fish went in. In pH water most common tank diseases cabbot survive, so the fish have little or no natiural resisitance to them.
I breed plecos and sometime use the RO/DI water when doing a dry/rainy season simulation. This is a few month process however.
Based on your post I would have assumed you already knew the answers. Otherwise instead of describing your tanks as soft water and hard water, you would have listed the actual parameters. Also, I would assume your tap water is fairly consistant in terms of the parameters. So it cannot be both soft and hard, it is one or the other or else in the middle.
So, I assume you are already doing something in one of those two tanks to change the parameters. I am curious re the actual parameters for both tanks and your tap.
UV is used for one set of reasons and the RO or RO/DI for another set of reasons.
Obe reason for needing a UV unit is often because one is in a higher risk situation regarding disease related issues than we normally encounter in the hobby. But more folks also use them to kill waterborne algae. The main reason for using the RO water is either because one's tap is too nasty or else it is too hard and has a higher pH then wanted/needed. We use the RO to lower hardness and pH. The pH is lowered because RO should have little or no KH whoch is what holds up pH.
filled with just bought a bunch of new fish feelings 
