The G250 needs to be isolated from whatever you bolt it to, and comes with a pin header for cable connections.
The G251 comes bolted to an annodized heatsink which isolates it (hard annodizing is a good insulator), along with screw terminals to make wiring it up easier.

Gecko sell the heatsink seperately for those who want to use the plug and play benefit of the G250, without having to worry about how they're going to isolate the driver.