Maylands has lots of flats but more of a homeswest presence than Mt Lawley. There is 45% apartments/units in Maylands compared to 25% in Mt Lawley, so with lots of units in Maylands you may be better off with a less common product/unit in Mt lawley. It also depends on the type of unit you are looking for - there is a big price range and quality on offer in either suburb, $350 000 up to a million. Do you want a one bedroom rental or a prestige apartment with views.
If you are looking for investment or just to live, but with the hope of growth you could do well in either, but Mt Lawley has a bit more scarcity and prestige about the location. Next door to each other, but Mt Lawley has the bigger cafe strip and shopping and is closer to the city, although not in any meaningful sense. The last 10 years growth was 9.1% for Maylands and 10.3 for Mt Lawley. There is not too much difference but the street location would affect growth figures as well.
In reading these pages it seems Maylands is getting a bad review, but I really think it is the specific areas - so take a look and see.