Hmmm....thinking out loud here.If you’re going for water clarification and algae, then 40 is fine. If you’re going for parasites than a 50 alone is not enough. I would add a 50 to the 40, or get an 80.
For anti-parasitic qualities, tank turnover should be at least 3x, preferably more. To achieve parasitic kill dosages you have to run water slowly through them (as you are currently with your 40W UV). So you won’t be able to achieve effective parasite control with a 50W.
It just depends on why you’re deciding to upgrade your UV.
If I did a 2nd 40watt and ran it inline: I could keep same slow flow, have a matching 2nd uv, be out $400 and have a challenge mounting it in the closet.
If I did the 80watt: I would need a dedicated pump in sump to handle the high gph, sell the 40watt, buy the 80watt and be out around $475. Replacing the 40 with the 80 would be better use of fish closet space.
Seems like in the long run, the 80 is the way to go.