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!
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.