Technically, this draft toss-up would be for the format of Dragon's Maze - Gatecrash - Return to Ravnica, and there are three sets worth of cards to look through there. But I'm going to focus on cards within the same sets, so we'll look at these two studs in the "DGR" draft format.
The Numbers (courtesy of Bestiaire.org)
Far // Away - #5 uncommon, #30 overall
Turn // Burn - #4 uncommon, #28 overall
The Analysis
First off, if this is your decision, you're excited. You have an option between two really powerful cards here. They're both removal, they're both flexible, they both run at instant speed. Both cards fuse for 5 mana. In the right circumstance, each of them has the potential to be a complete blowout. And even in the wrong circumstance, each of them is a small piece of removal (Away and Burn, respectively).
So what's the difference between them? Well, Far//Away is made valuable by its ability to get through things like hexproof or regeneration, since you're forcing the player to sacrifice, and it incorporates one of my favorite effects: Unsummon. Turn and Burn is more useful as a combat trick, when you can Burn one creature and Turn another to make it susceptible to an easy block kill. Turn//Burn is also a bit more reliable, giving a guaranteed kill of one creature as long as you can cast both parts.
I think they're both really good cards, but my pick is going to be Far//Away. While fusing either card gives you a great effect, the individual pieces of Far//Away are better in my book. An Unsummon effect is always a positive, and there are plenty of circumstances where an opponent only has a single creature, where Away will do everything you need it to do. You can also always use Far during combat to save your own creature while using Away as a kill spell. Turn and Burn are good individual effects, but not great. The value of Turn//Burn is really in destroying one targeted problem enemy for five mana. Far//Away can regularly do a good deal more than that.
The Verdict
So what's the difference between them? Well, Far//Away is made valuable by its ability to get through things like hexproof or regeneration, since you're forcing the player to sacrifice, and it incorporates one of my favorite effects: Unsummon. Turn and Burn is more useful as a combat trick, when you can Burn one creature and Turn another to make it susceptible to an easy block kill. Turn//Burn is also a bit more reliable, giving a guaranteed kill of one creature as long as you can cast both parts.
I think they're both really good cards, but my pick is going to be Far//Away. While fusing either card gives you a great effect, the individual pieces of Far//Away are better in my book. An Unsummon effect is always a positive, and there are plenty of circumstances where an opponent only has a single creature, where Away will do everything you need it to do. You can also always use Far during combat to save your own creature while using Away as a kill spell. Turn and Burn are good individual effects, but not great. The value of Turn//Burn is really in destroying one targeted problem enemy for five mana. Far//Away can regularly do a good deal more than that.
The Verdict
Far//Away