I can't figure out if there's an actual difference between these.
The first is simple present, passive voice.
The second is present continuous (also called present progressive), passive voice.
The resulting sentences are very similar. Because of context, the reader will assume the same set of facts in both cases, and won't end up with different interpretations.
Can I use them interchangeably?
Here, I believe the answer is "yes".
Is one more correct than the other in my example?
Let's modify it first, to see if that changes anything:
As the construction worker was loaded into the ambulance, he said "Don't worry, I'm fine."
As the construction worker was being loaded into the ambulance, he said "Don't worry, I'm fine."
There are some cases where you must distinguish between "present continuous" and "simple present", and the meaning will be different depending on which you choose. However, in this particular case, I believe the meaning is similar. The choice is based on more subtle factors. If it's spoken dialog, where you wish to be very abrupt and to-the-point, you could quickly say "is loaded". To match the style of the surrounding sentence, which is simple and direct, then again "is loaded" works. To indicate the man was loaded into the ambulance quickly, even instantaneously, then nothing progressive could have happened.
Alternatively, when writing journalistic prose where the sentence is more "complex", or where the progressive tense is already being used, you might choose progressive instead. If the process takes a longer time, and multiple events are taking place, then the progressive could be preferable.