I would gladly live in Mauville City. There's more behind this than liking the apartments themselves. There's more a feeling of liking the city itself. Wattson himself is no less of a draw too; he's a friendly sort of person, and I think the city itself has much the same charms of home anyways.
The shopping available just downstairs is really neat. There's plenty of places to dine as well as there being even a few trainer oriented facilities as well! As a Pokemon trainer I'd probably enjoy Mauville as a place; mostly because it's also centrally located as well as anything, there's an easily accessible route to another city just south of there; cycling road doesn't mandate any walking through pokemon habitats. I could easily and quickly get used to living in such a city.
As for rents; I doubt they'd be too significantly high. Given Mauville's location and population; there's probably not super high demand. The fact that there are multiple vacancies there also implies that perhaps the city ISN'T that desirable of a destination. But with two adjoining cities a safe route walk/ride away, I feel like it's nestled in the right place to be a nice and peaceful place to live.
That said; I wouldn't say no to a place in Mossdeep or Sootopolis, but both are island cities and are slightly disadvantageous, being in the waters of Hoenn, rather than the landmass of Hoenn.
Lilycove as well as Rustboro are also nice places to be.