At my school, when I was doing GCSE's they kind of broke it down like this.
- If you want to do something science related, or feel like you can get the grades, do triple.
- If you don't want to do something science related, or just want to ease the pressure a bit, do double.
And anyone who did BTEC was kind of under the category of 'they seriously don't want to do science, but took BTEC because they have to', which may have been unfair, but considering the number of lessons I walked past and they were on the laptops (and knowing the people, likely doing absolutely nothing), I don't know.
I don't know if you guys even have the option of doing BTEC (last year at my school they didn't offer it, so take from that what you will), but that was pretty much how they broke it down.
Now, I'm in year 12, so I finished my GCSEs last year (having done triple) and I think that if you are willing to put in the effort to do your best, and you want to, then do it.
If you want to do double just because someone else wants you to, then don't.
I understand that GCSEs are getting harder, I have a sister currently in year 10, so I can see the worry that it'll be more than you can deal with, but everyone has to do the new spec, and that is an important point. Grade boundaries are drawn up based on the results that everyone gets, so it's not like people that could have got A* before are going to get getting 5s.
All in all, do what you want to do. Don't do it because the school want you to, do it because you're thinking about your future and what you want to be doing for the next two years. The above word vomit was my way of (hopefully) trying to help that process.
Good luck!