I guess you have general knowledge on the difference between "は" and "が".
If it rains everyday and only tomorrow's weather is fine, in this case you could say "明日が".
For example, you can say; 最近は毎日が雨で、明日が唯一の晴れなので/明日だけが晴れなので....".
In general however, another day could be fine weather. Therefore, if I SIMPLY look at your two sentences, I would chose the second one.