I'll leave the choice up to you, but I can offer some advice regarding which champions benefit from high sigs and which only require a gem.
Doom: a great option for a gem. He's the kind of champion that you need to put either one or 200 sigs into. I wouldn't really recommend dumping all of those sigs into him, but a gem? Great value.
Abs Man: really solid awakened ability. Your omni-morph forms last longer (which is great) and you get a really sizeable heal (also great). Unlike Doom, this one is worth pumping full of sigs.
Tigra: if you know how to play her and have a bunch of sigs wasting away, she's also a great option. A fantastic champion that becomes much more fun to play at high-to-max sig. But also very tricky to learn how to play.
Diablo: if you run suicides, he's well-worth a gem. If you don't, the sig ability won't matter in 99% of matches you fight in.
Comments
Doom: a great option for a gem. He's the kind of champion that you need to put either one or 200 sigs into. I wouldn't really recommend dumping all of those sigs into him, but a gem? Great value.
Abs Man: really solid awakened ability. Your omni-morph forms last longer (which is great) and you get a really sizeable heal (also great). Unlike Doom, this one is worth pumping full of sigs.
Tigra: if you know how to play her and have a bunch of sigs wasting away, she's also a great option. A fantastic champion that becomes much more fun to play at high-to-max sig. But also very tricky to learn how to play.
Diablo: if you run suicides, he's well-worth a gem. If you don't, the sig ability won't matter in 99% of matches you fight in.