Yeah, Kill Bill is definately the most effective of the two. In addition to the great points already made about how the extreme close up of the battered bride is pretty relentless in the sense it's forcing you to mantain your gaze on that sort of visual, you could also talk about the engima that is Bill.
Being the title character, Bill is very clearly pivotal to the plot (even to the very casual audience). You could talk about how the decision to only feature Bill in such an ambigious way (just using his voice, brief glimpses of his hanRAB when he wipes away blood of Beatrix's face etc...) and how that ties in with the rest of the first volume of Kill Bill and how Bill is this shadowy figure for a substantial portion of the film's narrative. You might also interpret the decision to introduce Bill like this as a way of making the character more sinister.
That idea of obscuring the face of an antagonist is used in plenty of movies (slasher films always have a killer with a mask for example) and this isn't always just to conceal identity, but I think obscuring an identity can also be a rather unsettling thing. You could also talk about te shock of the bride in bridal gear and how the gore and blood strewn across her is a shocking contrast to what you might ordinarily associate with bridal gowns and weddings etc... She's also very vulnerable, being pregnant. You could also say that the fact she's in her wedding dress really exaggerates her femininity, and therefore might make her to be seen more vulnerable (depending on what you think).
As for "Gladiator", I can't remember exactly but I think the opening shot was a battle scene perhaps?
Even though the gore and the violence in Gladiator is extreme and very heavy, I think the audience sees it as more of a fantasy, taking into account it's expected context. What I mean is that the audience knows what to expect from the title, and I'm sure they have a good idea of what went on back in those ancient fighing arenas from history books and the like.
In my opinion, Kill Bill is the most effective opening. Not only for those reasons I said before, but because it seems a lot more about the individual and it helps rally you to her cause. I think the most important thing about it is that fixed, unmoving close up on her face. You see every ounce of fear and terror in her face.
