I personally lean stronger towards the latter than the former of the two, I don't have the time to put in to practising for a specified race each week or even to attend weekly and consistently. I like unplanned shenanigans with cars that are closer to stock as I find them ultimately more entertaining, especially on a wheel where you are trying out a few different cars and just hooning around really.
Though I fully understand the pull of an organised and regulated series, I was loving them during the summer, there's nothing better then going from racing something like stock 458s (IMHO one of the most entertaining cars in GT5 even if everyone says so
) and then someone going 'hey, lets have a Samba Bus race around the ring!' and having some utterly ridiculous race which just turns into hilarity before turn 1.
My internet is a problem right now, but timing wise I don't have time to practice for a series and run a motor in, or set up a car for other series, and such. Could I make time to just have some casual shorter races at a specified time each week? Probably.