What is XLSX?

A modern Excel workbook, which is really a zip full of XML. Unlike a CSV it knows what a number is, holds multiple sheets, and keeps formulas and formatting. It's what people mean when they say 'send me a spreadsheet'.

What is TSV?

CSV with tabs instead of commas, and the better choice more often than people realise. Tabs almost never appear inside a field, so the quoting rules that make CSV parsing miserable largely evaporate. Anything with prose, addresses or European decimals in it is safer as TSV.