Both are excellent operating systems....I have both on my desktop computers (one is 8.1 Pro, the other 7 Pro) and each has it's good points. With Windows 8.1 - the one thing I would recommend is purchasing Start8 ($4.99).
The only issues you may encounter that are "problematic" is some hardware/software on the market do not have drivers or are not supported under the newer operating systems.....I have a "graveyard" of printers that have not had drivers for Windows XP, Windows Vista, Windows 7 and now Windows 8......if you have older devices - check to see if there are drivers available.