Hey, I went to JMSS so can't comment too much on EBSS. I do know a few things though:
1. JMSS starts in year 10 while EBSS starts in year 11
2. I think extended investigation is a compulsory subject at EBSS
JMSS specific
3. You are encouraged to do 2 sciences + 2 maths (methods + specialist, they only let you do further if you're really struggling with maths) + 1 English (lit, lang or mainstream) + 1 subject of your choice (they encourage a humanities subject but you can negotiate too). However, this isn't a hard and fast rule and you can negotiate with the teachers and explain why you'd like to deviate from this. I know many of my friends did that.
4. They also offer accelerated subjects for methods, physics, bio and chem.
For methods, you have to sit a readiness test towards the end of the year you've been accepted and ~50 kids are accepted into methods 1/2.
Chem and physics have their own pathways which you can find in the handbook if you're accepted
Bio has no prerequisite to do 3/4 in year 11. You can just skip right to it.
Also of note: to do 2 3/4's in year 11, you need to have around a 90% average (calculated from all your yr10 subjects) but this can be a bit lower but it'll need some persuading :p
I hope this has helped you! Take it with a grain of salt as I graduated last year so they might change some things around