cm: If you compare the weights, an educated guess regarding which is the more substantial model should be pretty easy.
Well, and then the SL-10 also came with a higher quality cartridge, which in addition was a low-output MC - but thanks to the integrated, defeatable MC pre-preamp stage the SL-10 nevertheless only required an MM input. Another difference is that the SL-10 clamps records between the lid and the platter, so that it can also be used in vertical position - and yet another the additional power input (for 12 V(DC)). The SL-10 also still sports the tracking force fine adjustment, which the SL-J3 doesn't sport anymore, iirc. And of course the SL-10 is a much more iconic table, not only being Technics' first linear tracker, but also the first T4P model.
That being said, the SL-J3 isn't bad at all - so while the SL-10 would be preferable, if both exemplars were in good condition, a fully operational SL-J3 might be preferable to a somewhat broken SL-10.
Greetings from Munich!
Manfred / lini