Speed and endurance-question about practice pattens

I have a question about practicing for speed and endurance. Consider two practice patterns: a. You focus on improving speed in 25m swims; i.e. you don't swim consecutive laps, but pause for seconds/minutes after each length. After some months you will have increased your speed for very short distance (25m), but you don't know your speed for longer distance. b. You focus on endurance by swimming non-stop for as long as you can, without regard to speed. So after some months, you will be able to swim some 1000s, in moderate or low speed. What I am interested to know is, which practice pattern will likely help achieve the other practice goal more? In other words, will (a) help improve endurance more, or will (b) help improve speed more?