If you're looking for a wall/staller, Umbreon is the Pokémon, hands down.
Umbreon has great Defensive stats, and access to reliable recovery in the form of Moonlight, and also learns Wish to make Wish passing a possibility. Toxic is a greta move for Umbreon, or any wall for that matter. It can learn Heal Bell too, healing the team of status ailments and giving it the edge in a stall war.
It can also learn Defense boosting moves such as Curse, and the Speed drop doesn't matter because it is already so slow.
Serperior doesn't even compare when it comes to stalling.
There are other factors though, such as how the typing will affect your team, but Umbreon is better overall.
