There were some unspecifics about your post.
Are you assuming it's an unknown conv. oil?
Do you know for sure, or is worded as an oil change that meets the requirements for your prius.
In summary, is the deal going to be for something that satisfies the recommended oil in the manual? Or is it deliberately worse and they will surcharge or upsell you to get what is recommended?
E.G. is the "free" is just 10w-30 or something like that when the cap says 0w-20?
I doubt they would put in the wrong oil as the dealer, but you wrote it as if that might be a possibility.
So assuming it's the right oil, but just bulk.
If I were in your shoes, biggest decision would be is it a hassle to go to the dealer?
Do they get you in and out quicky? Is it pleasant and you don't get a hard sell on other services?
The other thing is you can change your mind.
So perhaps you go once for the first change , and then you can decide based on your 5k experience if you go back or not at 10k and so on.