The main differences are:
Odoo IoT requires additional hardware that you order from Odoo for subscription (IoT Box). You can use the Odoo Direct Print app with any machine that has access to all needed printers
Odoo IoT works only with Odoo Enterprise
Odoo direct print can print documents automatically and is configurable in a flexible way (Delivery slips, shipping labels, other reports…)
Odoo Direct Print works with any remote or local (USB, Wi-Fi, Bluetooth) printer and connected to any OS (Windows/Mac/Linux)