It also depends on your choice of neck. A lot of people seem to like the Ash+Maple combination for a bit of added brightness and snap, while Alder+Rosewood is the most popular pairing, offering the well known bass tones on most recordings.
Swampash tends to be lighter than regular Ash (they tend to use the woods from the bottom of the tree), where Alder would sort come up the middle on average, but can also range from light to heavy itself.
You're likely paying the extra $100 to get a nice grain, which you can choose not to seal with as many - or any - sealers. The J4 we built for Remco Hendriks for example, is a single piece of Swampash which was color stained before getting a rubbed on oil finish, with the complete bass barely tipping 7lbs when fully built. Alder is better sealed, from the advice I've been given.
In this case, I don;t think it matters. As far as I'm aware, Sire uses Asian body woods (as opposed to imported Alder and Ash), which are heavier in general. While overseas on sourcing trips, I was given the choice between US woods and Chinese woods and there was no question about paying more for the US woods, which were consistently lighter in blank form, like 2lbs on average.
I would suggest hitting the Sire megathread and ask people if their Ash bodied basses were light, average or heavy and decide from there.
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