Both heat water, in different ways. A solar thermal collector heats water directly, and it turns a much bigger share of the sunlight that hits it into heat than a PV panel turns into electricity: roughly 60 to 80%, against about 20%. So for the same patch of roof, thermal makes more hot water.
The trade-off is flexibility. Thermal only heats water. It can't run your appliances, charge an EV or earn export payments, and once the cylinder is hot on a sunny summer day, the rest of its output has nowhere to go. Electricity from PV always has a use: you use it in the house, store it in a battery, send the surplus to the immersion through a diverter, or export it under the Clean Export Guarantee.
For a new install in Ireland, PV with a diverter is usually the more flexible choice: it heats water on sunny days and saves or earns money the rest of the time. Thermal can still make sense where roof space is tight and hot water use is high.
If you already have a working solar thermal system, there's rarely a reason to take it out. Keep it and use the rest of the roof for PV. A diverter then adds less, because the thermal collector already covers much of your summer hot water.