Definitely recommend Ealing Broadway, many more facilities there if you need them compared to Acton Main Line plus you have the advantage of the Central and District lines too. Plenty of options near the station at Ealing Broadway as well, I've not been outside the station at Acton Main Line so I have no idea about what's there.
I might also recommend the London-bound platform at Bethnal Green, for spotting the 345s and other mainline trains. I haven't done that there for years, but it used to be incredibly busy and especially so in the peak periods for the amount of trains you'd see.
Westbourne Park's good for GWR and Heathrow Express, and LU of course, but that's it. I would still pop down there and have a look if you haven't been before, it's a good spot but not if you want to see 345s. Hold on, wait, I'm thinking of Royal Oak! Like yourself, I'm not from London, but that is still quite the rookie error!