So, for Rakdos, who will win?
While you might say that “target blocking creature gets -1/-1 until end of turn” doesn’t sound like a big deal, it can indeed be so. While it’s not the most impressive mana sink ability around, if you have open mana, it can actually save you a burn card later. It also lets you save valuable removal cards like Dreadbore or Hero’s Downfall for something more threatening. It also means that you can get through just a little more often.
His second ability is actually fairly relevant, too. Say you have one card in hand and it’s not anything you’ll use on this turn. While you may not want to tap out to use Rix Maadi’s ability, making a player lose 1 or 2 extra life can indeed be relevant (it will be more in the future). He’s not the best of the guildmages, but you have to watch out for him.
However, this older Guildmage has a very interesting second ability. For 3R (3 colorless, 1 Red), you can make a 2/1 red Goblin creature token with haste onto the battlefield, which you then have to exile at the beginning of the next end step.
In Commander, you can make a lot of little hasty Goblin guys to annoy your opponent, certainly. Infinite mana equals infinite tokens, as well. But what’s most interesting about this ability is that you can create chump-blockers on your opponent’s turn. You can also create potential problem hasty attackers on your own turn. Still, though, the ability is a bit more costly, and you have far more effective removal in the formats in which this Guildmage is legal.
Overall, Rix Maadi Guildmage is a bit more useful than his older counterpart. While he’s not flashy, the Return To Ravnica version is a bit more flexible in combat situations, and has a useful ping ability. From a Constructed standpoint, I have to go with the new kid on the block here. However, in Commander, the Rakdos Guildmage is actually a Goblin-spewing combo piece for infinite mana shenanigans.
But there is one more thing about Rix Maadi Guildmage that gives him the slightest edge. In Commander, while it's hardly a staple in any particular deck, players lose life on a regular basis. So his second ability to make a player lose an additional life for only 2 mana is fairly efficient.
Winner: Rix Maadi Guildmage, just because he's slightly better in Commander.