There are just a couple of differences between the JS900 and the JS1000. The first is the country of origin of the wood for the body. The JS1000 uses "American Basswood" which is generally accepted to be slightly superior to the "Basswood" of unspecified origin that the 900 like the 100 is made of.
The other major difference come in the form of the hardware. The 900 has the regular edge bridge whereas the 1000 has the "Lo-Pro" edge bridge. These are slightly different to one another but in my and several other folks' here opinions neither is really "better" it's just a matter of personal preference.
The 1000 also has some fancy electronic goings on in the form of a coil tap and
high pass filter on the tone and volume pots respectively, these aren't included on the JS900 but can easily be retrofitted. Teh coil tap will let you get more "strat" type sounds from the guitar. When you roll off the volume on a guitar, the sound tends to get a little warmer as some of the high frequencies are attenuated. The high pass filter can be engaged to prevent this and make the sound a little brighter and clearer when the volume is rolled off.
The JS900 does however come with
Chrome hardware which many folks consider the "proper" colour for hardware on a JS
As far as value goes, I think the JS900 is a great guitar full stop. £700 is about the standard sort of level I'd expect to pay for a second hand one in fantastic condition
Hope this helped a bit
