Pros -

Cheap - only one sensor and servo needed

you can use one sensors to monitor a wider area and there is a tool built in for this. Since one sensor is used to monitor a sweeping area you do not need to compensate for variations in sensor accuracy from one sensor to another.

Reduces effort and time - one sensor means one set of script. Using multiple sensors you need to write script that interlinks these and each must have cust script to make up for variations In accuracy between sensors.

Cons- since there is only one sensor it cannot see in every direction at all times. For the radar to be useful it must have a wide sweep otherwise its really not better than a stationary sensor because they have a beam about 6 inches wide it can measure from

Decided which one is best- for a very small robot radar is awesome. The larger the robot both width and height means larger blind spots and a radar is not as useful. My Omnibot for example must use 6 sensors to cover the blind spots at the corners and sides and areas in front close to the floor so it does not run over small objects.

Simply put;

Reaction time is drastically decreased if using a servo where as accuracy is increased tenfold if using multiple sensors.

Take the Ping Roam script as an example. While the sensor is at position 10 of the servo it isn't checking position 50 or 90, so any collision in the half a second it takes to move and check the sensor may not be detected - as can be seen in the testbot video on the ping roam topic.

The advantages are cost (both financial and with regards to ports used).

Personally I always use fixed position sensors and use multiple sensors when required.

The advantage of having them attached to the servo is allowing for a greater area to be looked at while your robot is moving. Therefore, you may not need to have as many attached. Although the sweeping motion will decrease the accuracy bit.

Using fixed sensors will give better accuracy, but you need more attached to cover the areas you want it to see

Thanks all for the very grateful help!