You should always train with RS gold slayer, it shouldn't take too time to achieve 70 points in a few days depending on how long you play every day.
A fantastic set-up for Slayer, that's relatively cheap is to utilize a Slayer Helm Neitznot (sp?) Helm until you can get the slayer's helm, Amulet of Glory, Fighter Torso (can easily be acquired within a couple of hours) Torag's platelegs or Verac's Plateskirt in case you'll be using prayer on your assignments as it grants prayer bonuses, Whip / Dragon Scimitar, Dragon Defender, and Dragon Boots as well as Obsidian Cape (unless you already have either a firecape, or skillcape).
Remember that is only a suggestion for your equipment it is always possible to change things up. For instance, you could swap your defender in for Obsidian shields if you think you require the extra defense boost. Alternatively, it is possible to buy OSRS gold use dragon plateleg's if you do not want to fix the armor of your barrow since it degrades during combat.