Okay, I've been waiting for someone to ask, but apparently everyone knows but me.
What's the difference between Nearest Neighbor and Bicubic in the preview pane?
Read the FAQs and search the forum before posting a new topic.
This forum is for reporting errors with the Training process. If you want to get tips, or better understand the Training process, then you should look in the Training Discussion forum.
Please mark any answers that fixed your problems so others can find the solutions.
More than that, it really depends on what you want to be looking at. Nearest neighbour will give you a (close to) pixel accurate output of what the model is doing. Bicubic will give you an output closer to what you are likely to see in the final convert.
My word is final