It obviously depends on, what you want. I have played a lot of 45 mans on PokerStars, and for me its a nice in between tournament format. It lets you experience all the phases of an MTT including final table play, but its still over in around 2 hours, which makes it much easier to fit into your schedule than a large MTT. Obviously a 6-man or 9-man is over even faster, but on PokerStars I have always gravitated towards the 45-180 mans for two reasons:
1) Lower effective rake
2) Hourly breaks
On 888 Poker they dont really have any multitable SnGs, but they have recently introduced an hourly break for all SnGs, which I think is a great idea. In general you need to look at, which games are available on the site, where you are playing, and then find the ones, which suit you best.