open-fronted are preferred as it is thought the horse can 'feel' it more if he touches a pole, but it is all down to personal choice, you can put on whatever boots you want.
Yep as others have said you can use either but open fronted if preferred as with the close fronted the horses feel less so get less sensitive and are more likely to start knocking poles (coz they dont feel it as they would with open fronted)