I lived in both Zone 2 and Zone 3 in London (4 years on and off, in Putney, Southfields and Colliers Wood).
Generally (as a guy) I felt safe walking most places in London at night, and on public transport. Zone 2 doesn't feel safer than zone 3, or less safe. It's more the certain areas that some people might say to avoid (but that's a different topic).
Zone 2 is not generally considered walking distance to Central London, although it might be, if you're closer to Zone 1 - for example, I've walked to Parliament from Earl's Court easily, but from East Putney I would not be particularly keen - it's a good 25 min on the tube!
From Ealing, you've got a decent way on the tube or bus for every trip. District or Central line, both will take you into town, and it's convenient for trains out west (overground). But if you're in town and want to 'pop home' to get something for the evening, it's a fair hike.
It'll totally depend where in Zone 2 the other option is. As seen on the tube map - Zone two covers an awful lot of suburbs, stations and possibilities. I'd try and find out where it is.
Of course, the other advantage of having a Zone 3 fare pass (eg buy a Zone 1-3 travel card) is that all your other trips to other Zone 3 locations would be essentially 'free'. And it's not that much more per week.
The other thing to consider is if the tube is down, you'll be taking the bus or overground. So if you know where you're studying (I assume that's why you're there) or where you'll be spending much of your time, play around with the TFL route planner and see how long it might take by bus, train, or tube.
One final small benefit I did notice is that the further you were from Leceister Square (in central London) the cheaper movie tickets seemed to be, so that's a plus ;)