Pent is better for a few major reasons.
First: Statistical comparison: 20/6 Erk vs. --/6 Pent
HP - 36.6 vs. 33 - Erk wins
Magic - 15.6 vs. 18 - Pent wins
Skill - 15.6 vs. 21 - Pent wins
Speed - 19 vs. 17 - Erk wins
Luck - 10.2 vs. 14 - Pent wins
Def - 9.8 vs. 11 - Pent wins
Res - 14.6 vs. 16 - Pent wins.
Basically, on average, Erk loses almost every stat. Marginal HP lead and a slight speed lead, but Erk will realistically not be 20/6 by the time you get Pent, so that direct comparison is pretty moot. Both Pris and Serra have claim to the Guiding Rings before Erk should get one, and if you're using Canas he probably gets one before Erk does since he's the only Dark user.
Pent also comes with an A in Staves, letting him use Physic. Unlike Erk, who starts at an E in Staves.
Pent also has a free no-effort A-support w. Louise, who is a solid unit in her own right.
Basically, Pent da gawd.
Pent from my last draft run:
http://i.imgur.com/fzec7an.jpg Bonus: Louise -
http://i.imgur.com/wlChGon.jpg
Both had an Agelic Robe thrown at them since it was a draft run and they best made use of the extra HP.