Saccharin and sucralose are both artificial and both equally un-natural
compared to sugar so no difference on that account! Saccharin has been
around a lot longer than sucralose but should only be regarded as
traditional if you regard the motor car as more traditional than the
horse. Saccharin was quickly adopted by farm and factory cider makers in
the 1900's because it suddenly gave them a means of making sweet and
stable ciders without having to go through all the proper care and
attention that is required to make a naturally sweet cider by truly
traditional means (i.e with many centuries of history) such as keeving.
As is usual and understandable, people take the easiest way out. After
100 years, unfortunately this practice has now become regarded in some
quarters as 'tradition'.
Relative sweetness values are given all over the internet eg here
http://www.elmhurst.edu/~chm/vchembook/549sweet.html
Sucralose is actually twice as sweet as saccharin so you actually need
less of the pure chemical. But I stress 'pure chemical' because if you
use the formulated retail forms they are all adjusted for bulk density
as I explained. The only sure way is to TRY IT on a small scale, choose
what suits, and then scale up. The amount you need will depend on other
perception factors eg the amount of acid in the cider.
If you add sugar (sucrose) a good rule of thumb is to keep no longer
than 2 weeks or re-fermentation will likely set in by then unless you
pasteurise. Sealing it will not prevent that, in fact it is most unwise
as it will give you a bottle bomb and maybe take someone's eye out.
Finally .. no, there is absolutely nothing natural about crushing apples
to a liquid juice, putting the juice into a more or less closed barrel
and letting it ferment under a build-up of CO2 which severely changes
the normal yeast metabolism to give alcohol by anaerobic respiration.
Only human beings do that. It does not happen in nature.
I am being deliberately provocative and trying to encourage people to
stop and examine their use of words like 'natural' and 'traditional' and
what they really mean in the context of cider, because I believe a lot
of doublethink and self-deception goes on here.
Andrew