That depends on your budget and lifestyle among other factors. One thing people wrongly assume, is that homes appreciate over time. Homes don't appreciate, land does. So, first look at the lot with the existing home. Is it in a good location? Is the size sufficient for your needs? What is the traffic and noise like? Given equal qualities and amenities for a lot with a home on it, and vacant land, then look at the existing structure. Do you like the layout of the home? Is the square footage sufficient for your needs? Do you like the style of the home? If yes, then you may want to purchase an existing home.
Does the home need updating? If so, and your budget and lifestyle allows for it, you may wish to start from scratch. By building your own home, you get to make all the selections that go along with the construction. Unless you have an unlimited bankroll, you'll need to be very conscious of your budget and selections, as its very easy to spend money!
Long term, I don't think it matters whether you build or purchase existing construction. Over time, the structure and interior will need updating and remodeling anyway. Look at the area (in general), the neighborhood and street (in particular). Do the neighbors maintain their yards? Unless you live on acreage, your value will be somewhat dependant on your neighbors homes as well as your own. Remember this: there are only 2 times you can choose your neighbors. When you purchase, and when your neighbors sell. Make the most of them...
Best of luck!