While merc is good to get at some point, I really don't agree with people telling others it should be their first char or things like that. In about an hour of training you're only really saving 9mins with her link skill. I don't think 9mins out of an hour is worth getting a character to 120 that you don't even like/want as a first or major priority. Get her link at some point, just don't rush it. I would say to make a zero for the card though, since there's less work involved and it makes that 15% turn into 20 or 25% (Forgot what zero gave) for just having the char.
On the note of the OP, I believe wild hunter is faster, form my own experience of course. Wild hunter has 100% stance and can move while attacking which is a awesome combo for getting around the map and clearing things quickly to Buy MapleStory Mesos. WH also has two turrets and one of them can two platforms at once which works well at places like drakes and RnJ. Angelic buster needs to balance skills to keep up the charge and her skills just aren't as "hold down to win" as WH's are. I've only gotten both a bit past 130 so this is based on that.
Well, I'd say Angelic Buster is more annoying... not sure if that qualifies as "hard"... they would both essentially take the same amount of time, just one of them has more annoying skills.
It would be better though to pick Mercedes since, once they get to level 120 they have a link skill you can assign to another character that will give them 15% experience increase... which in turn makes leveling everything else 15% easier.