The Off-Peak Day Return from East Croydon to Harpenden has restriction code B1 (not valid before 09:30), so there are no evening peak restrictions on it. That costs £24 at adult rate or £15.95 with a Railcard.
Contactless PAYG would cost you £16.20 on the outward journey (assuming you touched in at East Croydon between 16:00-19:00) and £11.80 on the return journey. Oyster isn't valid as far as Harpenden, in case you were wondering - and contactless PAYG doesn't allow any Railcard discounts to be applied.
Therefore, regardless of whether or not you have a Railcard, you are better off using a conventional return ticket. The calculus would change if you were to return the next day, as an Off-Peak Return would be more expensive (£42.70 / £28.40).
Of course, that is based on this being the only journey you're making that day. If you'll be making other journeys as well, contactless may be more worthwhile as the off-peak daily cap of £32.70 covers all Zones 1-6 travel after 09:30 (including the evening peak). A paper Off-Peak Day Travelcard costs the same and can be Railcard discounted, but would not be valid for the kind of 'reverse' day trip you're proposing - it would no longer be valid to return to London after you reached Harpenden.
