If it's *just* for the purpose of spotting secrets, then it doesn't matter much, I guess? Since most of the time your members will stay close to one another when exploring. I'd put Wits on the main character, cause that's the one I will be selecting and moving around most of the time.
But Wits does have combat purposes so it's best if you put Wits points on whoever benefit the most from it in combat. If your casters have Savage Sortilege, their spells can crit and Wits would increase their crit chance with spells. A dual-wielding bruiser with high Wits and Strength and Scoundrel can do a lot of crit damage too.
Another factor that you might want to consider is that Wits increases Initiative, which affects character position in combat queue. From my experience, it's a good idea to have one character with really high Wits, to make sure that that character will always go first in most, if not all, encounters. If you have a character who specializes in crowd-controlling or setting up combos, you could pump Wits on that character. A character who specializes in aoe burst damage can go first too, but usually it's a good idea to have someone else to set things up first, manipulating environment and/or combatant positions and so on, so the other character doesn't have to do anything aside from dishing out damage when his turn comes.
In my playthrough I put a lot of Wits on my assassin Sebille, so she could burst down any squishy critical target right off the bat (usually mages).
