The second ability is an expensive pump effect, 3G to give a target creature +2/+2 until end of turn. The fact that both this and the first ability can be played at instant speed is certainly important. Activated abilities are far more difficult to counter than cast cards, so Gruul Guildmage is an okay card overall. In an aggressive deck, both abilities can work, especially if mana flood becomes a problem in a given game, so he’s still a decent card, if only good back in his day. He’s a bit slow now.
Also, Skarrg Guildmage‘s second ability involves lands, like Gruul Guildmage’s first ability to sacrifice a land to deal 2 damage to target player. The good news here is that for 1RG (1 colorless, 1 Red, 1 Green) you make a target land you control become a 4/4 Elemental creature until end of turn that is still a land. You can do this as you enter combat. Potentially, this 4/4 land will deal a lot more damage to a player, especially if given trample with that other ability, than sacrificing that same land would do. Yes, it then becomes vulnerable to creature removal and being destroyed by blocking creatures. But it survives otherwise to stay a land at the end of the turn and going forward.
Gruul Guildmage may have been playable back in the days of original Ravnica block and the abilities are properly costed for what they do. But Skarrg Guildmage has two very underrated and powerful abilities for a far lesser cost that can potentially do far more damage. While he’s been in Standard, though, he’s been relegated to sideboards. It seems that there’s a lot of untapped potential in this creature. While he never became that viable in competitive play, he's clearly the superior card.
Winner: Skarrg Guildmage, by a considerable margin.