+1 on what olddog said. There is a measurable delay in time required to sense a change in AFR the farther from the port the sensor is placed. Some EFI systems have a compensation factor you can adjust in the tuning software but there is no way to change the fact the sensor is slower to respond the farther from the exhaust port that you put it.
One of the fixes for the problem is to take the flange on the headers (between the engine and side pipes) and move the flange back an inch or so. After you have moved the flange back make a stainless square block that will fit between the headers and the side pipes. Mill out the center of the block leaving a large square opening. Weld an O2 sensor bung in a horizontal position.
Assemble the headers and side pipes with the stainless block and gaskets between them. Tighten everything up, screw in your sensor of choice and go for it! Your sensor will be where the calibration engineers wanted it, in terms of distance from the exhaust port so warm up and signal delay will be "normal" in calibration engineer terms.
A nice benefit of this approach is the sensor is protected from road debris thrown up by the front tires and the side pipes look "normal" without any wires or plumbing going into the collector area.
If you haven't chosen a Wideband yet definitely look at
14 Point Seven (<= clickable) wide band offerings. Although they use the same Bosch sensors you can get anywhere they have the absolute fastest electronics for sensor signal to ECU time available anywhere today. Their pricing is quite competitive also.
Ed