[AQ]

I will assume that you have a UV filter on both lenses to protect the front surface of he glass. If not get these first.

I am sure that you know a polariser will reduce reflections and also in some circumstances darken a blue sky considerably. It will act as ND filter as well so you get quite a bit for your money

I think only you can say which lens to buy for. What do you usually take pictures of . Which lens do you use most . why do you want a polariser .

The 70-300 lens uses a 58 mm filter and the 50 mm lens a 52 mm filter. You could get the 58 mm size and a step down ring to use it on the 50. That would save some money.

As to where to buy from I got a polariser from Hong Kong for about 50% of the UK price. There are a lot of fakes from HK for things like memory cards and batteries. I assume that you can get fake filters from there as well. I do not know how to tell then apart but the one I have works OK which is good enough for me.