On second thoughts
Definition 1 of 1
Pro Tip 1/3Hesitation Indicated
Used when someone is hesitating on a decision and considering another option. 
I was about to paint the room blue, but on second thoughts, green might be better.
Pro Tip 2/3Casual Tone
The phrase often has a relaxed, informal tone and is used in everyday conversation. 
Let's go out for dinner. Although, on second thoughts, let's just order pizza.
Pro Tip 3/3Decision Revision
Implies that the speaker has thought more deeply and wants to change a previous decision. 
I'll take the job offer. But on second thoughts, I value my free time more.