The difference between the two sentences is when we say عندي we says about something we have it but we don't have it in the moment like ex: you talk with your friend and he asked you have you a dog? You will reply : عندي so we use عندي for speak about something we have it but isn't with us for moment or we have it in the past. But we use لدي When we have something and it's with us in the moment so we use لدي for present