Slayer is really worth it. With higher slayer you have more monsters to choose from. And these monsters drop good items at middle and higher levels. And it's better to be training 2 skills at a time more than one. Basilisks are one of the best exp training spots in the game, and they drop a fair amount of runes while killing them. And training there is fairly cheap, a load of lobsters lasts you 30 minutes. And in that 30 minutes you could and will have more than 3k worth of items(runes and such), probably about 100k worth of items.
I myself will be getting all my 99s in combat thru slayer. It breaks the boringness of training with just one monster.