Never sign a rent to buy.
If you prefer to rent. Then rent. And when the time arrives to purchase then shop the entire market for the best home at the best price. If you sign a Lease with the Option to Buy, there is a chance you will become tunnel visioned and not even want to shop the market. Chances are there is a better deal out there than the rental you are living in.
And the same goes for a seller. Rent when you want to play the landlord role. And when it is time to sell, then shop the market for the best/qualified buyer that is willing to pay you at or above market value. An owner looking to sign a lease with the option to sell is not confident enough that he can sell it after the rent agreement expires. And trust me, you don't want to own anything that a landlord determines to be an unmarketable home.