Oak Park is where my own traditional Catholic church is (Washington and Ridgeland, 10:00 Sunday mass is a great one, authentic gregorian chant) and I ride the Green line to get to it several times a week. The Green line is not as bad as the Red line, in my opinion, riding them both. There are fewer really disturbed seeming people on the Green line, mainly good working people are the rule. The Green train stations are well maintained and light, not like the Red line.
These might be some things to watch out for, when you check it out for yourself.
Do you have to go for just those two choices? The Brown line is quite civilized. I live in Lincoln Square. You can get some nice house deals right now in Albany Park toward the end of the Brown line.
The train ride here is pretty important to the quality of life, especially if you're a kid and a non-driver. Good luck!!!
Evanston is immediately to the North of the city.
Both connected by the EL train.
From Oak Park can take Eisenhower (I-290) into the city or major arterial roads.
From Evanston can only take major arterial roads.
Evanston has Northwestern and is right on the Lake.
As per safety, it is relative to one's own experiences and expectations but always be diligent regardles of where you live. Travel to both cities and do some drive bys at different times of the day to see how you feel about the proposed neighborhoods you plan on living in. Visit the local police department as well.
I lived in Oak Park and Evanston and both are relatively safe but if you walk around or ride public transportation you have to keep your wits about you at all times, a cell phone is handy too.
The best advice has already been given: Walk the area yourself, at different times of the day. Look for graffiti, or people hanging out on street corners, boarded up buildings, these things say a lot about the neighborhood and what goes on there..
I grew up in Evanston and have worked in Oak Park.