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Hi Cumbrian
Whether to get 1.25" or 2" is dependant on your eyepieces you intend using. As I gather from your sig you only have the stock EPs that come with your scope and they are all 1.25" then there is your answer. In case you are not aware the filters screw into the bottom of the EPs, they are threaded to take them.
The next question is which filter. OIII filters are primarily used for Palnetary Nebulae which ususally emit stringly in the OIII wavelength. They can also help with some details of emission nebs such as Eta Carina, M42 etc but I have found the UHC filters work better as they admit more of the other wavelengths from these nebs.
For a start, I would go the UHC filter, or with the Televue, the Nebustar. I have found that cheaper filters have an annoying habit of causing the stars in the field to be split with a little red dot nect to all the stars. I would imagine that the better quality filters (TV, Astronomik etc) do not do this, but I have no first hand knowledge that this is the case.
Remember that filters will not make nebs brighter (you need a bigger scope to do that!) but will increase contrast and block some of the light pollution from city lights.
Malcolm
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