As new tech terms appear and evolve, they tend to be spelled separately at first, and over time become more closely joined, either with a hyphen or no space at all. This evolution is occurring with data set (dataset) at the moment. According to Google’s Ngram viewer, data set was dominant from the outset until 2013, and authoritative sources like dictionaries and style guides (e.g. APA) reflect this, with some like Wikipedia giving dataset as an alternative (not incorrect) form.

However, in the past 15 years the trend has sharply reversed, and now Google’s corpus shows that dataset is almost twice as popular in British and global usage; Cambridge now gives only this form.

Data set was still holding strong in American usage until very recently (see Ngram and change the corpus to American English to see the graph). Dataset is poised to equal or bypass data set soon in popularity even in the States. Dictionaries are slow to reflect such changes; the AHD (American Heritage Dictionary) still gives only data set, while Merriam Webster gives neither.

You can expect this shift toward dataset to be reflected in dictionaries and style guides within the next five to ten years, and I expect that they will vary in which is given as the main entry, but most should soon recognize both forms.

Both are common enough that in my opinion you may choose freely between them, as long as you use one consistently in any particular document. There is certainly no difference in clarity. However, in the prenominal adj. position, as in dataset development, it seems logical to me that if your preferred style is data set, you would hyphenate here: data-set development, and if you prefer dataset, then it would be dataset development.