I've done a few sportives, but got sick of them - too many wannabe racers. After one bawled me out for stopping at a give way with a blind junction I decided bother with any more. If you want to race, mate, get a licence and stop playing at it.
Clubs vary - my local (Christchurch) has several slow and pottering type groups. The proble is usually smaller clubs where everyone gets lumped in together, causing frustration for all. Some clubs are relaxed, some a bit keen.
Sportives are good for introducing you to a new area and giving motivation of something to train for...if training up is your bag.
The CTC and Audax UK do lots of rides, the crowd is relaxed and more friendly. They also seem to be less gear-obsessed - lots of steel frames and scruffy blokes. When you talk to them many are the real deal who regularly do 400k or 600k events...not some bloke who reads C+ and dresses like a Power Ranger. The events also have advertised speeds. If you go to fast, the controls are not open and you have to wait!
Ride it or milk it!