175 or 250 watt transponder?

Bottom line: If you want to fly above 15k in airspace where a
transponder is mandatory, then 250W is required or you will not fly
there. Otherwise install whatever you want. As there is little
difference in price as well as in power consumtion, I don't see why you
would want to go for the weaker unit, though.

A side note: If you want to stick with a mode C transponder, then there
are a lot of used ones available in Europe pretty cheaply, as Europe is
mandating mode S now and everybody must switch. If on the other hand you
want to buy a new unit, then go for mode S. It doesn't cost much more,
it draws less current and it will have a much better resell value.