Advantage of TX mixing: No need to buy a mixer. Just get a good TX and it will pretty much last you forever, on all your robots.
Disadvantage of TX mixing: You have to keep changing models on the TX if you have different robots with different trims and mixes.

Advantage of plug-in mixing: Inverse of the TX's disadvantage. Plug and play, don't need to change stuff on the TX mixing-wise.
Disadvantage of plug-in mixing: It's just another bit to buy, and another bit to go wrong. Case in point, Alex (owner of Bruiser) at this year's Insomnia was using a plug-in mixer. It stopped working properly halfway through the event, so I lent him my TX instead, that has inbuilt mixing.

My advice? Just get a decent TX with two mixes and memory for multiple models on it. Should be all you need.