With only the belt, you cannot objectively set the
traction/pulling grams, with the
extender you can control that aspect.
The belt is handy, if you are going to be moving about, but still want to hold you penis extended. Advisable to remove either device after max, 3 hours, let things "recover/rest".
I bought both. There are situations where one my be preferred wearing over the other. Either can be worn under clothing, in public.
Its one of those choices where, if you can swing the bucks, get both. No one can predict which one, if either, YOU will prefer over the other.
When it comes to the learning curve, the rod
extender may be easier. Your call on that one.