Bypass regulator. No question.
A return from the pump is just an external bypass (other pumps have an internal bypass). This external bypass keeps the pump pressure from getting too high, and it does cool the pump better than an internal bypass, but it doesn't give you the other advantages of a return regulator.
1. A return regulator lowers the temperature of the fuel all the way up to the regulator, not just up to the pump.
2. A return regulator lowers the pump amp draw and increases pump life. When using a dead-head regulator, the pump is making maximum pressure all the time. When using a return regulator, the pump is only forced to make the regulator set pressure at idle, and only a few psi more at full throttle.
3. A return regulator decreases pump noise.
4. A return regulator keeps fuel moving towards the front of the car. Moving fuel has momentum which helps offset the g-force at launch.
5. When using a return regulator, you can set the fuel pressure even when the engine is not running.
6. When using a return regulator, the bypass at the pump (internal or external) should be disabled. This leaves one less thing to go wrong.
If you are not sure if you want to run a dead-head or a return regulator, we offer one that can function either way:
http://www.robbmcperformance.com/products/regulator.html