I got my first character to light 325 in RoI. Do I have to use legendary marks and strange coins to get to light 350 at this point, or is there something faster and more active that will give me the drops I need?

@ksck26: There is a post I made addressing this exact question in detail here. But the short of it is either do two things, grind Legendary Marks to buy Vendor Gear or wait until Friday when Xur comes. If you have Strange Coins buy the 4 pieces he sells if they are for different gear slots and infuse them into your gear. It is much cheaper, at only 13 Strange Coins for Armor and 3 Legendary Marks for Infusion than 75+ Marks and 19-23 Coins for Weapons. If you have a stash of 75+ Strange Coins you should be in business. Buy a Ghost from a Vendor and trade in a Legendary Engram for Class Item to Shiro if you have one, he gives out 350 class items. Finish the story and Quest for an Iron Artifact at 350 and you should be in business. Then start hitting up the Heroic Playlist. As of the patch today that will drop gear up to 385, so just grind that or Archon's Forge until you are high enough Light to do the Raid, Normal is recommended 370. But go read that guide, it has much more detailed information than this.

Edit: You could just go jump into the Siva playlist and rank up with Rares if you don't have the materials, that's the Active, but slow way. The way I'm talking about only takes a few minutes to jump you up 25 Light per slot. Either way is fine and will get you there. Buying your way to 350 is faster though. But if you don't have the materials go ahead and play some Siva Strikes.

Your best bet is the SIVA Strike playlist, use Three of Coins on the bosses and just pick up engrams. You'll get a good bunch of drops, then when you can, start playing the Heroic Strike Playlist. You'll get what you need no problem! Remember, as well, grab Legendary Marks where you can, via the daily, crucible matches, etc. You can purchase better gear or use them to infuse better light gear to your stuff.