Depends on what you are measuring. If you are after what figures you put down on the road, then the lesser of two evils would be the wheel dyno. The hub dyno, like a Dynapack dyno or the likes, do not have the mass/inertia of the wheels to use in the measurement. They may use some figure to compensate for that, but realistically it is not as accurate as a direct measurement. A wheel dyno on the other hand will measure the power directly as it is put down at the wheels. However if you are just monitoring differences or changes to your car, then either dyno will do. Preferably the wheel dyno would be better in my opinion since when running on it, you car pretty much will behave like it will when on the road. Hub dynos take out the problem of tying the car down/wheel-spin though, which is handy...