Pontypool is probably quicker and easier to get to id imagine as is it a faster road once you leave the M4? Cwmbran is probably around 10-15 minutes once you leave the M4. I only did Cwmbran as they had more services stopping there and knew the car parking was okay. Its probably been about 3 years since ive done it (it was 175s not MK4s) and it was definately free back then. I prefer to take the train the entire way and always do if im staying overnight or away for a weekend. Its only the fear of mssing the last connection in Cardiff late at night that tempts me to drive part way!
I'll have to check next time I go, but it is certainly a quick road around the east of Cwmbran. It is slow through New Inn, but there is little traffic at 06:00.
I am not the only person leaving a car in Pontypool car park overnight. There are always a few there when I arrive in the morning. In December there were some very frozen examples.
Of course getting a Mk4 back to Pontypool was nice because of the capacity, but I don't consciously aim for them. Just depends on what time I finish work. Ironically, this will be 1V62 once teaching starts. Previously I aimed for the 16:30 if possible because the 17:30 was often 2-car and that really would be cosy and staying in the office until 18:00 is not attractive if the schedules allow otherwise.