Both can be very tedious to level, but I've found Mercantile to be more simple. Don't haggle, sell everything you can, and sell items individually instead of always in stacks.
With Speechcraft, eventually you'll increase everyone's disposition, but you can also stop before they're maxed and work on lowering it again -- careful if you get it below 20 though, or they could attack you. Fame and Infamy also affect the disposition of all NPCs, so it'll depend on what you've been doing up to that point. There are also a lot of meta-gaming ways to work with Speechcraft, which could affect your roleplaying -- for instance, one exploit is to draw your weapon before speaking with someone to lower their disposition, but you could also find a roleplay reason for the weapon being drawn. Plus, personally the Persuasion minigame jars me from the roleplay experience :shrug:
Strictly for controlling it to level, yeah, Speechcraft. But from a roleplaying perspective, Mercantile IMO.
I'm glad you made some of those points, for whatever it's worth.
Yes-- speechcrat, from a pure metagaming perspective, is the easier skill to control. But I LOATHE speechcraft as a major, just because if you pick it as a major, you have to pay attention to it -- "I don't want a level up yet, so I can't play the mini-game." or "Okay, now it's time for a level up, but I've got everybody maxed, so I have to do the drawn weapon exploit." or any of that other metagaming stuff that completely destroys my immersion.
I want to just play the game, and for that, I find Mercantile to be a far, FAR better major. It increases at a nice, slow, steady pace, no matter what. Just pick it and ignore it.