Citizenship

Hello, I am hoping to get some help here. I filed my N400 at the end of December 17' in NYC. Currently the "estimated case completion time" is October 18' (from September 18' initially but still solid). My green card is expiring in June, and today I went to USCIS to get an I-551 stamp, I was told I cannot get the stamp unless I filed six months before expiration. I was a few days late ...

The rep at USCIS suggested me to file I-90 to renew my green card in order to get stamp since the estimated time is likely to delay, but when I looked at the history in this forum, looks like NYC has processed to Sept 17' filed case and in that case I may be just a few months away.

Any advice here? I dont want to spend $540 and get interview notice the next few days, but on the other hand dont want to lose the capability to travel for more than a few months.

Is the estimated case completion time includes the time from interview to oath? Or it's at best getting the interview only? If I am more than likely to not get citizenship until 19', I will file I-90 now. If it could happen within 18' then I would try to save the money.

Thanks for the help and advice in advance.