I've owned both. My impressions with flats fitted:
Jet II - is a well made bass and no need to upgrade tuners, pu's, etc. Heavier with a slab body and nice feel to the neck. Meaty tone, (sometimes referred to as heft).
Jag - a beginners bass, some folks upgrade tuners and pu's. Lightweight and comfortable, narrow neck it's tone is OK-ish.
Eventually I settled on a Squire Mustang. Better build, (no upgrades necessary), preferred tone, similar narrow neck to the Jag.
The feel of the necks is quite different due to the slightly wider nut on the Jet which is nearly 40mm. The narrower Squire neck, about 38mm, takes a bit of getting use to. I actually preferred the wider Jet neck. Those 2mm are noticeable.
My other SS is the Hofner HCT Violin which has a 42mm nut. If you haven't tried one please do. You'll end up either loving or hating them as they appear to attract strong reactions.
Hope this helps.