Thunderbolt has 10% chance of Paralysis.
Zap Cannon has a 100% chance of Paralysis.
If you are wanting you Eeleketross to be a haxy Pokemon then go for Zap Cannon but remember it is not very accurate...At all.
If you just want a good Special sweeper than Thunderbolt is for you. Personally I would go for Thunderbolt but it completely depends on what role you want your Eeleketross to hold and maintain.
Hope this helps! ^.^