501hdv vs a 503?
 

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

The difference.
 

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.