I have researched the cost of UV lights and have now a decision to make. Should I go with 30 watt fluorescent BL's that are 36 inches long or perhaps I should choose the other option which is 20 watt fluorescent BL's that are 24 inches long?

I recommend making the largest UV bank that will fit comfortably into your working space. Here in the states the 48" tubes and components are often cheapter than 24" and 36" tubes so when I compared costs it was actually less expensive for me to build a 48" unit than a 24" one. This may or may not be true in other locations.

Even if you never plan to make really big prints having the extra space will allow you to make several exposures at a time if you desire. With my large unit, for example, I can expose a two or three 12X20 negatives at a time. This can be quite an advantage with alternative processes because of the long exposure times involved.