Which writing style for uncertainties?
Yes, there is an agreed upon standard and it is specified in the Guide to the expression
of uncertainty in measurement (GUM), section 7.2.2, prepared by the Joint Committee for Guides in Metrology (an ISO committee).

Both forms are equally valid, and people use them indifferently. The first form is just more compact and it is particularly useful in tables, abstracts etc.

For an uncommon usage of the first form, see also this answer of mine on Physics SE.