I was recently asked which comparative form is more correct, using so or as: “England is not as hot as France” or “England is not so hot as France”. It’s an interesting point as they are both possible, so neither is really more correct – and one of my reference books covers it in one simple point “After not, we can use so … as instead of as … as.” This doesn’t give you much room for discussion! So, do the two forms differ?
When a rule is presented as briefly as this in guides, it leaves room for interpretation. This is the case with many supposed rules in English – where alternatives are possible and there might not be a strong reasoning behind one choice or another. In this case, “not so” is comparative, the same way that “not as” is – to be more precise, they are negative comparisons – we only use so in place of as when it follows not or no (a no form would sound a little archaic/lofty but is still possible, such as “There are no people so happy as bakers.”).
When it comes to choosing between as and so here, it is more a matter of style and personal preference. From my perspective, so is a little less formal – though in general if you are more used to saying as I would stick with it – there’s no real benefit to starting to use so instead. However, not so is quite common in shorter sentences with a fragment, such as “It’s not so hot.” This would be found more in spoken English, and more accurately (or commonly) so is replacing too, rather than as, for emphasis rather than comparison, although in certain contexts it might form a comparative statement.
Comparing positive to negative comparative statements
For an additional question, it’s worth asking why we’d choose either “not as” or “not so” over a simple positive comparison (e.g. “France is hotter than England.”). Choosing the negative comparative over the positive changes the emphasis. The positive form, “France is hotter than England” puts more emphasis on France, and would be used to discuss France, while “England is not so hot as Paris” (or “not as hot as…”) puts the emphasis on England, highlighting more specifically the climate of England. So the choice would depend more on the focus of your sentence – the one you lead with is the one you are more likely to be discussing.