If I may... the Squier J Mascis is the better of the two. No two ways about it.
I tried a couple of the VM JMs when they first came out and ended up buying one of the Jaguars! They just felt a bit, well - cheap. The Mascis on the other hand doesn't feel like it's a cheap guitar, although neither does it feel like a 'proper' Jazzmaster. The pickups are beefier (sort of P90 in design apparently) and suit 'rock' sounds, so no trad Jazzmaster tones either. Also the bridge is Gibson-style Tune-o-Matic, not the proper rocking Fender offset type. This should be replaced with a roller bridge at the first opportunity as the strings tend to snag on the saddles if you use the trem a lot. Oh and speaking of the trem, it has been relocated, a la CP, so there is also a difference in feel from a proper Jazzmaster and fewer of those 'famous' overtones.i
The neck- it is chunky, but not unpleasantly fat Gibsonesque- I can swap between it and a Rickenbacker 610 (toothpick thick) without any problem - and the fat frets make it a very nice thing.
That said, I want another Squier Mascis in which I would like to install a set of vintage-sounding Jazzmaster pickups. A compromise perhaps, but I have had rather negative views on the Japanese Fender Jazzmasters after having owned two less than impressive 90's ones, the only other option for me would be an AVRI or a vintage, and both are expensive options. You can always Tippex over the Squier logo!