The best thing to do - is to do what's fun for you!
Levelling up and progressing in your second full season will be easier. If you play casually, then there will be plenty of people on these forums who will be very happy to power level you. That way, you don't have to go through the grind of levelling up to 70 (if you find it tedious). Effectively, you will catch up with your previous P180 reasonably quickly, so will not really be missing out. You can also dip in and out of playing the season/non-season toon - so best of both worlds :-)