The First Order will follow the Falcon.
When they land at Takodana, Finn asks the same question, and Han says (thanks @DVK for tracking down the quote from the novelisation):
“Why are we here again?” [Finn] asked as they started up a wide, curving stone staircase.
“To get your droid on a clean ship. Do you think it was luck that Chewie and I found the Falcon? If we can find it on our scanners, the First Order’s not far behind. … The galaxy’s full of watering holes, but nothing like this place. It’s been run by an old smuggler named Maz Kanata for a thousand years. Want to get BB-8 to the Resistance? Maz is our best bet.”
Han knows that the raiding parties on his freighter were aware of the First Order’s hunt for an orange BB droid and may have seen him taking off in the Falcon. They also don’t like him, and have likely tipped off the First Order to his escape.
If the First Order know to look for the Falcon, then he needs to put Rey, Finn and BB-8 on a different ship.
- Changing ships makes it less likely they’ll be followed back to the Resistance base.
- It reduces the chance of Rey et al. being caught in the process, or of Han being arrested for smuggling fugitives. He probably doesn’t want to be arrested, especially by the First Order.
- Han is not keen to return to the Resistance if he can help it. If he has an opportunity to avoid it without too much risk, he’ll take it.