Would it be worth buying a hue hub?

I have a both hubs and stability has been great. The custom driver by @blueyetisoftware is superb and allows increased control in ST that you don’t get with the official driver, such as the ability to import accessories e.g. motion sensors, which can then be used in automations.

[ST Edge] Philips Hue LAN [BETA] - Developer Programs / Writing Edge Drivers - SmartThings Community

For best response times you’re best having a sensor and bulbs connected to the same hub however. I’m assuming your bulbs are non-hue and would stay on the Smartthings hub? Unless you intend on expanding on Hue devices it would be worth weighing up the cost of the Hue hub against a new motion sensor.