Pianos and electronic keyboards are both useful. Which one you buy depends partly on your interest. If you are really looking to just play piano, then there is no question that a piano is what you want. Not even the best electronic keyboards are as satisfying to play as a merely decent piano. If you want to play other sounds or use it with a computer, then you need an electronic keyboard.
If you are going to take piano lessons, you are best to get either a decent piano or a quality keyboard with a piano-like keyboard.
Playing piano is a physical task that requires strength and agility. If you want to play basketball, you’re not going to develop the strength an agility you need if you practice with a Nerf basketball because it’s so much lighter than a real basketball. Real piano keys are made of wood and have to move a mechanism (the action) made of wood and metal. They have weight. To play them, your fingers will need strength enough to move them easily.
But basketballs are not the weight that they are by accident. A Nerf basketball is hard for an adult to manipulate because they are too light. Pro basketballs are meant for grown men to handle. Their weight enables development of the agility they have with the ball. Similarly, the weight of the piano keys enables pianists to develop agility with the piano. This is not only about playing faster or with greater coordination, but also about controlling the quality and volume of sound for expression. It is difficult to develop strength and agility of any kind on keyboards with light keys.
There is some middle ground. Many keyboards today are made with weighted keys and touch sensitivity. This means that the keys provide a resistance much like the wooden keys of a piano, and also that, as with a piano, the greater force with which the key is struck, the greater the volume of sound produced. Below are some recommended pianos and electric keyboards for piano lessons.
Pianos
Yamaha & Kawai make some of the most popular upright pianos for students. I highly recommend them as their quality is pretty consistent. There are many models, but currently the Yamaha B series, Yamaha U1, and the Kawai K2 pianos are recommended. Some other brands that have produced good student pianos are Baldwin, Mason & Hamlin and Story & Clark. These companies and others made pianos for a variety of markets, though, and not always were the instruments well made and durable. It is a good idea to have a tuner/technician or piano teacher check out a piano before you buy. They can help you avoid buying an inferior instrument or one with problems.
Keyboards
As mentioned above, for piano lessons, a keyboard needs to have weighted keys and touch sensitivity. It also have should built in sound and at least a sustain pedal. My best recommendation is the Casio Privia PX-160. I would also recommend the Yamaha P-125 and the Roland FP-30.
Which is better?
Aside the issues of learning to play, there are other practical issues. Pianos, if well maintained, tend to hold their value pretty well. I sold my Kawai upright, which I’d bought new while in college for $2300, after 25 years and a lot of playing and transporting. It was the only model of piano I’ve ever encountered that had a defect in the finish. There were some faded spots which, it being black, were quite noticeable. Otherwise, it was a wonderful piano. I would have sold it for $2,200 if not for those faded spots. Even with that, I got $1900. Pianos do need to be tuned and kept in a climate controlled environment. In Houston, the latter is generally not a problem, since we all want our homes to be climate controlled environments! Tuning should be once a year and costs $100-125.
Keyboards are not going to hold their value, but are likely to be quite a bit cheaper upfront. They also do not need much maintenance, although the likelihood of needing a repair some time during several years of ownership is much higher than for a piano. A keyboard will also have a much shorter life than a piano. A piano can easily have a life span of 50 years or more. A keyboard will be doing very good to last 10.
So, upfront a keyboard is cheaper and lower maintenance, but a piano is a better investment.