If you are selling within 5 years, just rent.

Anyone who gives you advice to buy or sell now is at best making a guess. If anyone really knew what prices were going to do for sure, they would sell everything they owned and buy in the place that they "knew" would increase the most. My advice is if you are going to listen to others, listen to those who put there money where there mouths are.

James,
This is a very valid question. When you sell and there is a new product on the market as your competition, many buyers would prefer new. If your condo has a unique feature then you have the attraction and can attract the buyers in any market, but if you are selling one of 100 condos that all look and feel alike and competing against new you could be in for a rough sell.
The other consideration is what the area will be like in 3 to 5 years. Mission Bay built out, the Transbay and Rincon Hills occupied and serviced it is going to be a very dynamic place to live. In any area of SF if the market is soft just wait it will pick up again soon.
People that are in trouble now bought with nothing down and need to sell or a lack of understanding of adjustable rates. If you avoid both of those pitfalls you will be in good shape and you will have enjoyed your new home for all those years!

James,

You should think about what kind of condo you want to invest in and what will the next buyer like about it. Will you go for one of the buildings with many amenities, or a loft without one single closed room, or the "fixer upper" where you can add some value by improving the inside of the condo. (there are not many such condos in Soma/South Beach but still a few)
I think that Southpark is a very desirable location and your risk of not beeing able to sell in a few years would be lower but your point about so many new buildings is very valid.
Good luck with your decision making.

This is a great time to be buying. Yes, the market will get a lot more condos. What is important to consider is that most of the buildings will continue to be more luxurious and will continue to push prices. Also, as the neighborhood matures, over the next 5 years, values will go up too. It's kind of like the Wild West - where there is risk in the new neighborhood. Once it becomes a fully built out neighborhood, prices will get much heigher.