Carlsbad and San Marcos are two interesting communities to follow.
Coastal Carlsbad has decreed 40 percent open space, has a desalination plant in the works, and is surviving the market slump in relatively fine fashion. It is also served by three school districts, which can complicate moving decisions.
San Marcos has exploded with growth, has a wide selection of newer homes, and has aggressive plans for a mixed-use "downtown" area via the Creekside Project. However, because it is more inland and fully in the San Marcos school district, home sales have been more problematic.
If you have long term vision, I would not fear a move to San Marcos at all. It is a town full of promise and first rate developments.
If you are in the market over a shorter period of time, you may be wise to stick with Carlsbad and homes that are in either the Carlsbad or Encinitas/San Dieguito school districts. These areas, I believe, will hold their values better in the short run.
I think right now the further inland you go, the more risk you bear in terms of what happens in the next year or so. I say that because there are more foreclosures the further inland you go. That's part of the reason why prices have been driven down (i.e., if the house down the street is being discounted by the bank, then other sellers need to compete with the bank who is willing to wholesale the home). If you look at the REO maps on our web sites, you'll see that there are a lot more REO properties in the MLS in San Marcos than there are in Carlsbad. Now, if you think we've hit bottom, then it probably makes no difference. But if you have doubts, then location is something to consider.
Finally, as long as you have a fairly long-term selling horizon like 7-10 years, it probably does not make that much difference, and, if there is a difference, I don't know of an agent that would be wise enough to make a prediction about which will be stronger. If you have a fairly short-term selling horizon (3-4 years), then you need to choose carefully comparing everything about two different homes. (e.g., features, amenities, schools, roads, traffic, etc., etc. etc.) One issue that is a really interesting one is the question of how close will be a new development when it comes time to sell. If there is a new development across or down the street, that CAN affect values if the market is flat. That's because the builder can discount or provide loan discounts, while the homeowner is in much less position to do so, and I can't tell you how many times we heard buyers say, "Well I can go across the street and buy a brand new home for the same price or just a bit more." We have a Crystal Ball (seriously), but it's a search tool, not a prediction tool.
PS: If you are a numbers person, you should check out the http://www.SANDAG.org web site which has projections for all the various communities.
To get a real sense of value in both areas, you may want to have an agent who is familar with both that can give you some sales history on properties that are similar to the one you are considering now. It is true that San Marcos has more planned growth (development) than Carlsbad does, but that comes with both pros and cons. You will be best served by working with an agent who concentrates in North County and has specific community information that can help with you decision. Information such as: specific school information (not just district information), planned development stats (commercial, retail & residential), tax base etc. Knowledge is power. Escpecially when it comes time to negotiate the purchase of a home. Arm yourself with a broker that has the service, experience and specific knowledge of those neighborhoods to negotiate the best price on your behalf. On a side note- all homes for sale in San Diego County are linked to our website, along with community information that is invaluable. Please let us know if we can be of any further assistance.