the basic idea is UV is only good for a couple of weeks if it's effective killing the free floating algae. But all that algae gets put on the pond bottom, unles your filtration is fine enough to catch it. Most here don't use UV as we have bogs and lots of plants; these two things are more effective and provide more benefit for the pond.
All that said, most lights have a half life anyway and once you reach it, the intensity lessens such that you get a lot less benefit. I'd go by any hours rating and work the math that way. I generally don't like overpaying just because it's a name brand but it is true; you do get what you pay for.
Good points so far.
The most important things are the socket has to match and the wattage cannot be more than the original or you can burn out the ballast or possibly melt the wiring.
As long as the part numbers match, you should be OK.
I would go for the less expensive one since UV lamps don't last very long anyway. I believe they need to be replaced every year. They lose their effectiveness even though they still light up.
UV lights are one of those personal opinion subjects.
Usually, when people use them, they have an algae problem which points to an imbalance somewhere in your pond's ecosystem.
Many of us prefer to address the root of the problem rather than fight the result.
I prefer a more natural way of keeping the water clear by adding lots of plants and having adequate filtration.
Too many fish can be a problem too.