Hi Laura
On the face of it either option would appear to be viable.
As a rental, your apartment would produce a decent gross yield and your financing rate is obviously very reasonable. Your proposed new PDH mortgage would also appear to fall within the Central Bank's 3.5 LTI/80% LTV limits.
However, as things stand, I don't think you really have a sufficient cash cushion to retain the apartment as a rental. One bad tenant, or even a run of significant maintenance issues, would put your finances under huge strain.
If I was in your position, I would opt for the tracker mover mortgage option and would let the apartment go.