Invest now. I've never heard of anyone being sorry they saved money in past years. Well, I guess I'm sorry I just saved it instead of investing it. I just put 80K of savings into the market this year, and I really wish I'd invested it sooner. It was supposed to be a house down payment, but it just kept getting bigger, and we never bought a house. We were already maxing out other retirement options that were available to us (401K & Roths).
One reason you might want to stay out of the 25% tax bracket (even if it's only the dollars over x amount that are taxed at 25%) is to take advantage of 0% capital gains tax rates. I transferred some non-retirement investments from American Funds to Vanguard this year, resulting in about 10K of capital gains. I am very cognizant of which tax bracket I end up in, because at the 15% tax bracket, the capital gains tax is 0%. If I'm in the 25% tax bracket, it goes for 0% to 15%. If I'm understanding all of this correctly. So I could pay $0 on that 10K gain, or $1,500. It's not just the last dollar at 10% more.