I can tell you that being rid of USCIS is a great feeling. After 40 months of waiting for wife's I-751 to be processed, she filed her N-400 (5 year rule). Four months later, both the I-751 and N-400 were approved after a combo interview. As far as the IRS is concerned, you still have the same obligations now (as long as you are a Green Card holder). Good luck on your decision. But, as I said, being out from under the thumb of USCIS is great.