From what I know about these fences, things will depend on your dog. If he has a high pain threshhold and sees something he really wants to get to, the electrical fence might not be deterent enough. Also as some one mentioned, these don't prevent other creatures from entering.
If you have your mind set on the e-fence, this might simply be something you have to try and see if it will work for your dogs. Making the existing fence dog proof might be a better long term solution. At least you'll be more certain that it will keep the dogs in.