list vs. concatenate

from time import time
class Timer:
    def start(self):
        self.starttime = time()
    def stop(self):
        print(time()-self.starttime)

def fat_catenate():
    s = ''
    for i in range(3000000):
        s += 'a'

def listomania():
    l = []
    for i in range(3000000):
        l.append('a')
    ''.join(l)

t1 = Timer()
t2 = Timer()

t1.start()
fat_catenate()
t1.stop()

t2.start()
listomania()
t2.stop()