first of all, welcome to birdforum!
Actually the ATX85 is a little bit heavier than the Kowa 883 with the wide angle zoom. But it's less than 100g, so can be safely ignored.
Since you seem to be in a position to try both scopes, I would recommend to try both and if you find an example that you like and that is nice and sharp at 60x (or even with the extender), to get exactly that example. Sample variation is unfortunately a thing.
The focus mechanism is also a major difference - some like Swaro style, some don't.