This decision is a factor of several things.
1. What rear gear ratio will you be running?
2. How much HP will you be running?
3. What becomes available in your price range.
The rear gearing is the first priority in my opinion. If your gearing is 3.73 or numerically higher then the 700R4 is the way to go so you get good performance and good highway mileage. If your rear gear is 3.40 or numerically lower down to 2.41 the 350 will be fine for both with the optimum rear gearing of 3.08 for a balance between performance and mileage. This is what I found out when I had the same choice but there are probably others that have different advice. Do some research on the web regarding both trans. The 350 is cheaper to buy and repair. With my rear gearing I went with the TH350.
If you are building a high HP/torque engine you may want to consider the TH400. These are harder to find but are great transmissions.
Good luck.
