if investing in mainline, I would suggest small homes in Malvern and Frazer, with the following reasons:
1) more companies in Malvern area, therefore, renters (who work for the companies) can easily commute daily. Daily driving distance to work is a big factor for professional renters.
2) lower property tax because of the city tax collected more money from big companies. Therefore, you save big money on property tax. Other areas in Mainline cost more for oeperation.
Besides, west chester is a good place to invest, too. Generally speaking, the west area of Philly are much better than the easter regions (South Jersey). On the west side, from south, Greenville, Chads Ford, Mainline, to Yardley, New Hope, are all collection of affluent areas.
While east side, on the Jersey side of Delaware river, Camden is like a cancer you need to avoid. cherry hill is ok, but it's a large area, I think the more east, away from Camden, the better. Although the traffic is horrendous, it's convenient to go to Philly. Mt Laurel is too far, but Moorestown is a very good place. Even though the house prices there are a little bit high, but you can always find cheap ones for renting and bring you the positive cash flows.
Good luck