Membership expires, therefore your member items would be useless. AC stuff (non-mem of course) are with you forever, and can be stored in bank. I might get membership to get non-member items (such as DragonBlade of Nulgath) or other offers (DragonClaw, Fury set, etc.). My opinion though, no hate .
Both of them are cool, With Ac you can buy Items that can be stored , You can buy bank space and etc, With Memmbership you can get AC items By Doing some quests , You can Create Guild , Can get Memmber Only Badges , Can accress Memmber only Places and get awesome Stuff (most of them are non-memmber so when memmbership is expired it wont be gone) , But as Xask said , Its better to add Money and buy Memmbership+Ac Pack :P
If you don't already have CardClasher I'd go with the third option and buy it from HeroMart; the class is great all around and it's one of the only classes that can dps more efficiently with a bot running it than a person (it's a skill timing thing).
Plus it's a non-mem AC class, so you'll also have it forever.