high side vs low side current sensing in isolated bench power supplies, does it matter?
I would say that it does not matter, but there are trade-offs for each type of current sensing. Assuming the voltage source is always DC:


High-side current sensing often needs a precision voltage divider to get within the common mode range of the op-amp/comparator. An option in some cases is an op-amp designed for high-side current sensing, but it may need supply voltage greater than what your sensing the current on. The sense resistor need to be greater then 1 ohm most times, and as high as ten ohms, thus loosing some voltage across it.
Low side current sensing is preferred in many cases where you already have an isolated source. The sense resistor can often be less than 1 ohm so voltage drop is tiny. The penalty is if a serious over-current or in-rush current exist. It can damage a low-ohm resistor, so it should be over-sized if possible.
As far as random oscillation goes, you can put a 100nF capacitor across the 10 K resistor of U1. In addition you have no bypass capacitors on your power supply input and output. At least 470 uF from U3 collector to PSU ground and from U3 emitter to PSU ground. One of the main reasons for using capacitors is to stop spurious oscillations. A 'loose' rule is 2,000 uF per each amp of current consumed by the load.