I had both a 993 cab and a 993 turbo( wish I still had it), and currently have a 997 cab...
"Better" does not really apply to the comparison. They are both great, just depends what your wanting. These are just different cars.
Beginning with the obvious, the 993's were the last air cooled 911's. In my opinion they produce a different sound.
My guess is that its more a matter of personal preference and budget. The 997, being newer, feels tighter and there are technological advances . Its also faster, and more expensive.The 993, being the last air cooled 911, seems to hold its value really well-so if you buy one smart, you probably wont lose money.
With the new model recently out, the 997 still has some room to fall, but, by and large, its a fantastic car.
If money is an issue, go with the 993, its cheaper, and in my opinion, a better "investment"...If money is not an issue, go with the 997, just do your research and buy it smart. If this is your first 911, you will be very impressed and happy with either one!
Cheers!