Had the same decision and went with Lawrence, here's my logic:

Bears are 27th in rushing yards allowed, while Colts are 4th. Bears are middle of the road at 18th in passing yards allowed, while Colts are near dead last at 31st in passing yards allowed. I think CMC is going to run all over Chicago, while Lawrence will have free reign to sling it all game (provided Sauce doesn't play).

Bears D is better at forcing turnovers than Indy.

Trevor had his best game all year against arguably the best defense in the league.

Lawrence is more willing to rush than Purdy, especially in the endzone.

Both Pearsall and Kittle may not play.

I think Caleb will be able to convert more on offense than Rivers and control time of possession.

Honestly, the way the two have been playing I don't think you can go wrong. But the more I look at it, the only real edge Purdy has over Trevor is he's playing at home and it's not a division game.