Weejianle, if you intend to take exams, then you cannot run away from a real piano. But if you just got started, you may not want to commit too much money into it yet. How far are you into piano now? If you are dead serious, then get a second-hand piano. You can tell the difference between the touch of a real piano from even the best weighted keyboards. It's more than just the weight and touch of the keys - it's the reverberation of the sound in the piano body itself in a real piano. Even my son can tell the difference immediately.
For $300, it's going to be hard to get anything - real acoustic piano or weighted keyboard. If you are not willing to commit yet, my suggestion is to go to piano studios to practice (rent it by 30 minutes block for beginners, if possible) - and if it is not located too far away from your home.