Thunderbird uses AVL engine built in collaboration with AVL corp. of Austria. This split engine is made of Aluminum to reduce the weight of the engine for better mileage. So I can say, the engine fitted in TB is not at all a bullet engine. For the sake of mileage and pickup they have compromised the quality of the engine. I am not denying that TB engine is not good. But if you are a hard core bullet fan, you will not experience the charm of the bullet.
Whereas the Enfield Std engine is made up of heavy cast iron both barrel and piston. The engine also has a additional lever which pushed the piston at slow speeds. A std bullet produced thump while TB produce noise.
Enfield bullets suffer from vibration coz of 350 cc single cylinder design. You can have a damper fitted to your vehicle to reduce the vibration. The damper costs around 400 rupees.
You can go for TB. It is definitely worth for long term but If you are used to traditional std bullet, TB may not suit you.