Each wheel bearing supports approximatly 1/4 of the vehicle weight. The fronts have more direct thrust on them.
Things to consider when making your decision:
How long do you plan on keeping the truck? If you plan on putting another 30k on the truck I'd just replace them.
When were the bearings last replaced? Over 20k...replace as they have already seen about 2/3 of their use.
Do you plow in the winter where you are? If so the 600 lb plow hanging off the front is not kind to your front bearings.
Do you travel places a seized bearing would be a catastrophe?
How concerned are you about ride quality, or do you travel at highway speeds much? Mismatched bearings will have a definate vibration to them on both front wheels. It may even feel like you need an alignment.
Normally you would replace bearings as they fail. In this case someone did you a favor and has offered to replace them because they didn't keep the sets together. You should take them up on it. If they don't feel they should pay for their mistake then you should consider a different mechanic.