I've stayed in both, I prefer the location of the Kensington vs. Maida Vale. It is especially easy to take the Piccadilly line from LHR to Earl's Court, and it is a 5 minute walk to the hotel. The rooms are pretty similar, if you get a renovated room at the Kensington (they have not finished renovating all the rooms yet.) The lounge at the Kensington in the basement level, so no windows. But the recent renovations have improved the look and feel of it. If you are talking about staying at the Kensington in the immediate future, be aware that the lobby and restaurant are under construction. The bar area is still open. In the past, Plats and Titaniums had their choice of breakfast in the Lounge or the buffet in the restaurant. Since the restaurant is currently closed (breakfast served in a conference room), that option does not exist. I asked at the Front Desk whether it would still be an option in the future, but they didn't know.
I would have no hesitation to stay at the Maida Vale again, I just happen to like the Kensington a bit more.