Is it worth the price difference? What are the benefits of the 503 if you are not exceding the weight

The biggest difference is gonna be that the 503 hdv head a more nuanced counterbalance ability. Instead of on or off its got three steps. Also its a higher quality head in general.
I own the 503HDV and it works like a charm.

I used to work for a company selling heads and legs.
If you can stretch for the 503 go for it.
it just feels free-er and better.