I’d say Penhydd, it works slow and fast so everyone gains and there is suitable payback for the climbs of which there are only two noteables (IMHO) The initial climb and the one after Frog Corner, the rest are short and not too bad.
If you are doing both I’d do the Wall then Penhydd so you end on a high, although the end of the Wall trail is great, the ride back to the trail centre is far from and not a great memory to take back if you are trying to get someone into the sport.
As already mentioned get down the Drop Off Cafe for Lunch it’s probably the best trail centre food stop in the country. Personally I like there pizza’s but it’s all good.