the clip in ones are good if you can hide the clips well.
it would be ok for the wedding if thats the first day you wear them or get them close to the wedding date.
i think if its windy then they'll be fine. you could always get a hat.
or you could use the extensions to tie your hair up prettily and the clip would be hidden well as long as you put them in right, which youd probably get tips from your hairdresser or someone anyway.
i heard that the ones you get done at the hairdressers can take up to 6 hours. it looks good, but whether it's worth the time or not, i don't know. someone else at school had extensions, but she used heat to stick them oonto the ends of her hair. she said it didn't take long, but then, she didn't have that many so quite a lot of her hair was its normal length, which didn't look good either. have fun x