哪句句子的文法才正確？

Both of therm are wrong.
Tenses:
If you want to write this sentence in your diary at night, both are correct in tense.
However, if you say this in the morning or in the afternoon, you should use 'is' instead of 'was'.
Agreement:
Always compare things with the same category.
Therefore, you should say '......, but less perfect than yesterday's (weather).'
NOTE:'weather' in bracket can be omitted after adding 's.
The reason why you are wrong may be because you turn Chinese into English directly. Please pay more attention.

Attention:
No comparison for adjectives such as "perfect", "excellent", etc. They are called absolute adjective.
"less perfect" is absolutely wrong!!!

2012-08-12 10:10:52 補充：
"less perfect" is wrong in grammar as there is no comparison for absolute adjective.

The sentence should be in past tense because it is a result of your experience.

The weather today was good, but no better than (the weather) yesterday.

2012-08-12 10:15:23 補充：
typo: ..., not better than ...

2012-08-17 10:13:27 補充：
Today's weather was good, but not better than yesterday's. (today's and yesterday's are nouns in possessive form)
The weather today was good,... (today and yesterday are adverbs)

Both are acceptable!

no. 2 is better.

However, we could rewrite it in better English.

Today the weather is fine but not as good as yesteray.

we can replace good with perfect if you prefer it to good

The weather today is good,but less perfect than yesterday.

The weather today is good, but less perfect than yesterday is correct!

第一句!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!