A few years ago nearly everyone in state schools did 'dual' (now called core science and additional science) and very few did separate sciences (triple). The A level courses were (and still are) designed to follow on from the dual qualification and there was really very little advantage to doing the triple - indeed it was arguably better to keep a broader range of choices at GCSE.
Then just a very few years ago, the government tried to encourage uptake of science A levels by giving every bright child a so-called 'entitlement' to study separate sciences (this was based on KS3 SATs I think - how it's done now I'm not sure). This 'entitlement' was notional only as many schools still didn't offer separate science GCSEs, but as a result more and more schools did start to; separate science GCSEs became increasingly taken up by the brighter members of the cohort and so has achieved a sort of kudos. I'm not sure the initiative will succeed in tempting more people to do science A levels as my view is that the reason people aren't opting for them is because the GCSE courses (all of them) are dull, and giving children more of a dull subject seems unlikely to persuade them to elect to do still more at a higher level.
Now, with separate sciences becoming more popular again, a few schools are making it a pre-requisite to go on the A level course (eg: need Chemistry GCSE to do Chemistry A level and Core+additional science won't do) even though the A level course is designed to follow on from the dual. There are also some rumours that for very competitive courses like vetinary/medicine, some universities are beginning to favour those with the separate science GCSEs (not sure if this is true or anecdotal).
So, it is important to do separate sciences if you might wish to go to a sixth form that mandates it and study science A levels. It should be noted that only a very sixth forms do make this stipulation and they are all either independant or grammar schools as far as I know. Otherwise I think the dual award and a wider range of GCSE options is a good choice, unless your child actively doesn't want to do any other GCSEs & would prefer to study the extra science.
If your school doesn't offer triple and you really want your child to do it - discuss with the school the possibility of learning the extra work independently and sitting the exams at the school (all the core and additional exams can be counted to either dual or triple GCSEs, so an extra paper (or sometimes two) in each subject is all that is needed to make dual up to triple.