Wolf = better body balance, agility, condition, WAY better speed, aggression, team work, with longer, earlier peak (if both purchased at the same time and the same age, Wolf's peak ends right as Solimar hits his, which is only about a season long).
Solimar = slightly taller, better response, jump, WAY better GK skills, defence, mentality.
Wolf becomes much more balanced overall, plus his 1v1 stopper special is more practical than Solimar's penalty stopper, imo. However, they're both useful because they're so different. If you're playing a team that works the short game and plays to feet into the box, use Wolf. If you're playing a team that plays in the air and shoots from distance often, play Solimar.
I normally buy both, with the American newcomer Huber as my third (who, once again, provides a different option, more Friedelesque, durable) or occasionally Fredriksson or the 6'7" tank Cuypers.