Neither Simon nor Lelouch would fair well in each other's series.
Simon isn't a great pilot, nor is he all that smart either. He's a hard worker and relies on his power to push toward victory, his endless optimism extends because and is the source of his power.
In a non-Lagann mecha, he'd be reduced to just a screaming faggot. Nothing more.
Lelouch wouldn't have a hard time outsmarting everyone in Gurren-Lagann but that's not saying much, everyone's dumb as a rock. His commander style isn't the kind of over the top larger than life inspiring star than Simon and Kamina are, so he'd have trouble keeping morale up especially because he'd be so distant to everyone on board the Dai-Gurren. Not to metion his spiral power would be abysmally low.
I think Lelouch would fair better in GL than Simon in CG, but both would have a nigh impossible time succeeding in the same way as their swap did.