If you're referring to boxes, the consensus that I've read is M10 is superior. This is also consistent with my observation. They'll both be in Standard until the same time, so the consideration, then, would generally be based on the "jank ratio"- how many jank rares there are in the set. Only a couple M10 rares are totally unplayable, and cards such as the five duals, the five planeswalkers, Baneslayer Angel, and Honor of the Pure further inflate the value of the set. Biggest issue here: M10 is currently in short supply nationwide. Boxes at brick and mortar stores may not be for sale at all, or may cost upwards of $100. If M10 isn't an option, I'd pick Conflux or Alara Reborn instead; Conflux's Path to Exile and ARB's Maelstrom Pulse are both highly relevant cards.
Conflux doesn't have many high-value cards outside of Noble Hierarch. Reborn isn't much better, with Maelstrom Pulse, Meddling Mage and some moderately good mythics.
going to shop. what box set should i buy?