Luke there isn't a difference as such. They are the same person, but the Warmachine / Hordes time line has moved on between release and now, and the Epic version is not a more powerful Warcaster, but rather the same Warcaster with different abilities.
For instance Cryx Warcaster Iron Lord Asphyxius, killed a Lich Lord and got promoted by Toruk to a Lich Lord. The War Witch Deneghra was killed by her twin the Cygnarite Hayley and has been resurected as an Undead and is now know as the Wrath Witch Deneghra.
As for different abilities / Warjack points etc, the Warcaster from what I understand completley changes how the same models work. Whilst the E. Severius may seem more powerful on paper, he is probably a little weaker on the field, or his feet is not quite as powerful, hence the extra Jack point. From all the talk I have heard about Warmahordes, PP have balanced the game fairly well, so a weaker caster will probably be able to field a little more support. I guess its all down to personal preference on the battle field. You may not win at all with one caster, but switch out to one that works better for your play style and the rewards will be reaped.
For instance Cryx Warcaster Iron Lord Asphyxius, killed a Lich Lord and got promoted by Toruk to a Lich Lord. The War Witch Deneghra was killed by her twin the Cygnarite Hayley and has been resurected as an Undead and is now know as the Wrath Witch Deneghra.
As for different abilities / Warjack points etc, the Warcaster from what I understand completley changes how the same models work. Whilst the E. Severius may seem more powerful on paper, he is probably a little weaker on the field, or his feet is not quite as powerful, hence the extra Jack point. From all the talk I have heard about Warmahordes, PP have balanced the game fairly well, so a weaker caster will probably be able to field a little more support. I guess its all down to personal preference on the battle field. You may not win at all with one caster, but switch out to one that works better for your play style and the rewards will be reaped.