Happy Holidays & Merry Christmas...
First you need to look at the location of the property. A new home in a not so desirable location is definitely not better than an older home in a desirable location.
If they are all in the same location, then it might be a case by case situation. You will need to look at how old the older property is, does it have good bones, whether it has good floor plan which might not be easy to change. Will the update to the older home bring more value to you than the new home? If you prefer certain features, are those available in the older design as well as newer design? - for example, older design might not have the open floor plan, vaulted ceilings, over sized bathrooms, etc, which will need more extensive remodel, and thus more expense.
I am not sure about Virginia, but for where I am in Marin, a older single family home is usually more desirable than a newer town home.
So I think the answer is not a one size fits all, but depends on the properties you are considering. I would suggest a good Realtor in your area who understand your market condition and what the homeowners prefer to have, the Realtor will be able to give you a much better suggestion depending on the house and the new development you are interested in.
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
You'll pay more for new, but you'll get new with less things to change/improve/fix in the near to mid term. You'll pay less for an existing home, but you typically won't the "latest and greatest" and you may have to replace appliances, etc., within a few years.
If you go new, definitely try and buy a spec. If you would like to negotiate the most you can from the builder, let me know. About 40 percent of my buy-side transactions are new construction and have gone through this many a times before. I also know of offers/incentives available that aren't shared with the general public.
Look at where the best location is for you, and where you can plan to live for at least five years. If there are older single family homes in the same price range as the newer towhomes and you are handy and can do some of the work, you may want to consider buying one of those.
Good luck on buying your first home!