解码和unicode之间的区别？

根据这个测试:

# -*- coding: utf-8 -*-

ENCODING = 'utf-8'

# what is the difference between decode and unicode?
test_cases = [
    'aaaaa',
    'ááááá',
    '?????',
]
FORMAT = '%-10s %5d %-10s %-10s %5d %-10s %10s'
for text in test_cases :
    decoded = text.decode(ENCODING)
    unicoded = unicode(text, ENCODING)
    equal = decoded == unicoded
    print FORMAT % (decoded, len(decoded), type(decoded), unicoded, len(unicoded), type(unicoded), equal)

.decode()和之间没有区别unicode():

aaaaa          5  aaaaa          5        True
ááááá          5  ááááá          5        True
?????          5  ?????          5        True

我对吗？如果是这样,为什么我们有两种不同的方法来完成同样的事情？我应该使用哪一个？有什么微妙的区别吗？