My height is 5'5", which of the Enfield bikes should I buy - Classic or Thunderbird?

Dude, don't go for Thunderbird. Thunderbird looks like bird. Go for classic, even 5.5 is enough for Classic, only purchase Classic, trust me, you will not regret

According to your height, the best Royal Enfield would be the Thunderbird 350. It comes with a low seat height of 775 mm, making it more comfortable than the 800+ mm seat height of Classic 350. It comes with a projector headlamp for clear low light visibility and has a huge 20 liter of fuel tank for easy long drives. It has been priced at INR 1.25, justifying the powerful machine, which produces 19.8 BHP and 28 NM of torque. It comes mated to a 5-speed gearbox. It comes with a front and rear disc brake, making it a good partner for long and comfortable rides.

Must Know Facts about Royal Enfield Thunderbird 350