Depends what you want it to do.

Hunting or target.

IMHO sub 12FPE I go .177.

Much wider range of pellets and ultra light to ultra heavy options.

.20 has simply never caught on and is not substantially supported by makers of equipment or pellets.

.177 has the edge for target and distance.

You could argue it has more clout but the .177 JSB predators seem to smack hard that not much gets away.

I wont be tempted by .20 and the new and 2nd hand market reflects its popularity anongst the air shooting community.

Its a no from me