D grade clan oath armor and the A grade apella set are very different from each other. The oath armor was made to help out newbies and are comparable to regular D grade sets. Apella, if I recall correctly, has purely pvp related bonuses (cp and a chance to reflect a debuff onto attacking enemy, and some other stuff i can't remember), but isn't really that great compared to the normal A grade sets - and to obtain it, you pay with (and ungodly amount of) clan reputation points + some adena, which is better spent on other stuff - unless you have a surplus of RP from having multiple nobles become hero in your clan.