There's lots of good advice here. Just a couple of additional suggestions: If you've been searching, and have looked at both new and existing construction, you probably already have some idea of whether you "need" new construction or whether something a bit older will work. Some people absolutely demand granite countertops, the big whirlpool bath, etc. Fine. Buy new. Others may want more established communities, or a location that already has been built out. Fine. Buy existing.
But that's kind of dancing around your question. You asked about which would be a "better investment." Two answers to that: it depends on the motivation of the builder (regarding incentives and upgrades) and the motivation of the seller of the existing home. A highly motivated seller is likely to offer the best value. (Assuming they didn't buy 2 years ago at the top of the market with an option ARM that they can't afford. In that case, it's likely they're upside down--owe more than the house is worth--and so can't discount the house sufficiently. But that's another story.)
Second answer: Bluntly, who cares? You're not an investor. You're not buying an investment property. You're buying a house to live in. No, you don't want to overpay. But you're going to be living there awhile--average is something like 5 years. Pick a place that makes you happy. Close to good schools, if that's important. Close to shopping, if that's important. Close to work, if that's important. One with a patio, if that's important. One with a his-and-her bath, if that's important.
Bluntly, again, you're not investors. And, heaven forbid, in today's market I hope you aren't speculators. There are ways to buy low and sell not-so-low, but that's not your game. Take a look around at all those foreclosures. A lot of them are the result of people who thought they were "investing"--at the peak of the market, overpaying, and financing it all with terrible loan products. Find something that works for you. Be happy.
Realtor: Long & Foster