Hi

The choice of whether to buy a single-family or townhome is really something that is specific to your family, is more of a lifestyle choice. Townhomes offer convenient living-you don't need to do exterior maintenance, etc. which may be the best option for someone who has neither the desire or ability to do these things (of course you could always hire someone in that case) Townhomes are all the rage right now, but I think that's because they are more affordable, and offer more square footage than a garden-style condo.
What will keep it's value better? A single-family I always consider to be the best investment, because you as the owner, can decide if you'd like to make an addition, change the color, plant a garden, etc.
Do you have your heart set on new construction? For some people, that's the American Dream. For other's, a resale home (especially in a market like the one we're in now) can get you some great values AND solid quality construction from the "pre-particle board" days.
Being first time home buyers, you should know that there are a number of programs that may be able to help you. And I just checked the MLS for the Ayer/Groton/Littleton area. Plenty of single family homes for well under 400K
My direct email is
Valerie@SNHhome.com and you are welcome to email me. I"ll help get you the info if I can...