If you need dig particularly badly, throw out Serperior and replace it with Leafeon. Doesn't matter too much if it's the starter, I've traded my starter out for a more useful Pokemon before too :P
Anyway in any case I'd still support Leafeon. This is because Leafeon's attacking stats (110/60) are still better than Serperior's ones (75/75). Also Leafeon holds the superior physical bulk, and it's special bulk and HP aren't too much behind Serperior's. Serperior does hold a much better speed, but Leafeon's is also more than enough for ingame purposes.
Since you're running offensive sets on both, Leafeon woud generally outdamage Serperior, and Leafeon's better moveset right now leaves it walled by less types. Serperior relies too much on 3 grass STAB attacks and Dragon Pulse as coverage, while Leafeon has a better coverage range with attacking moves of 3 different types.