Groves is much better technically but I don't think there is much difference in what they've achieved in their careers.
Groves lol. It is weird though. Groves is more well-rounded and as an effect much more technically sound than Lemieux, but Lem is better at doing the particular thing he does best than Groves is at anything (if that makes sense- I haven't had any real sleep in a while so I could be phrasing this poorly). As a result Lem has much more unfavorable matches than Groves does, but I also think there are instances where Lem can either beat people Groves can't OR beat other opponents more easily/convincingly than Groves would (if they were the same weight).
Groves, he has better head movement, counter punching, footwork. Both maybe punchers but Lemieux is basic in comparison to Groves in terms of boxing skill.