You should look in both areas if these are the areas you prefer. Your goal is to have positive cash flow and a good investment...this type of property can be found in either city so please don't limit yourself to just one city.
But I forgot to mention in my first comments - before you invest, talk to your financial advisor to make sure investment property fits correctly into your overall financial plan. Most advisors suggest that real estate make up no more than 15% of your investment equity (I agree!). Best of luck!
I agree that the money spent on HOA fees would be better spent on a mortgage. In addition, the complexes with high percentages of rentals are probably some of the worst bets.
There are some great complexes in and around Sonoma County, and those that are close to Sonoma State University I think represent a good perennial rental market due to the presence of the college.
If you are in Sonoma it would probably be equally convenient to you to find something in Southern Napa as parts of Santa Rosa or Rohnert Park. You might find some good deals in single family dwellings north of Sonoma in Agua Caliente, or over the hill in Santa Rosa, particularly on the west side of town where prices are a little lower. I had clients who looked in both Napa and Sonoma find some good values in Sonoma and Santa Rosa over the last year or two.
I just pulled the August sales data for all of Sonoma County, which I will present in a blog post later this week. You can look at sales data from the last year on some of my previous blog posts, with links to all of the 2010 reports so far. My reading of the data vis a vis Napa is that Napa contains a bit more distressed property and has not quite seen the bounceback that we have (such as it is.). We have a bit more diverse economy, where as the Napa economy (beyond agriculture and tourism) is more linked to the east bay and American Canyon.
Note--August Sonoma County sales figures will be posted by the end of the week, so you can check back. I can also pull condo data, and the same analysis for Napa County, if you would like to contact me for that information.
If you can, I highly recommend passing on a condo and looking for a single family home.
Also, I recommend buying property close to home if you intend to manage and maintain it (go with Sonoma...).