Besides just being set in the same historical context, both involved a high ranking Roman Soldier falling in love with a Christian slave girl, and Poppaea is interested in him to further complicate things. And both are films I like.
Mercia and Lygia
Comparing the females leads, I find Lygia was stale and two dimensional, she's a common example of how Christian good girls in these kinds of films often lack any real depth, she also comes off ridiculously naive at times, and the actress I didn't find to compelling either.
Mercia however while having some of those tendencies still comes off as a much more real character, I really felt for her watching the film, the Lesbian dance scene is not just there to make the film "Sexier" it's an important scene, as I haven't seen the censored version I can't even imagine how the movie could have gotten around it. You can see in the scene that Mercia is being tempted, she is struggling, melting a little to use their words, then she draws strength from the other Christians singing and it's a very powerful scene, and my favorite of the film.
If only it didn't reinforce negative perceptions of Homsoexuality it'd be perfect. It is worth noting however that the most extreme Ant-Gay Christians tend to believe it not even possible for a truly saved person to even be tempted by homosexuality. That having that inclination at all is a sign of being no longer even eligible for Salvation.
I suppose there is room in my head canon to see Mercia is someone who in-spite of her revulsion to this obscene pagan display, would not disapprove of loving intimacy between two loving believers of the same sex.
I just noticed both male leads are named Marcus? Names sometimes escape me.
Now it's reversed, Fredric March's performance seems not very well fleshed out, going from admirable to a total dick from scene to scene without much warning, and at the end I didn't buy his conversion at all, it just seemed to come out of nowhere.
In Quo Vadis Robert Taylor's character is far from the most brilliantly written male lead, but at least I buy his character development, we see his view of the Christians slowly growing so his finale conversion scene seems very believable, and it makes a nice ending.
Charles Laughten was a great actor, but h didn't get to do much here, what he does is very good but I'd still have liked more.
Peter Ustinov gives a very compelling performance despite my historical objections to how Nero's depicted (He was a Tyrant but a component one, mentally unstable but not completely irrational) I still loved the performance a great deal, and it makes the film very entertaining.
I don't agree with this historical view of her, most of the bad things said about come from historians of the senatorial class much latter. Josephus actually knew her and paints a very different picture, and there is no basis in any early Christian traditions for suggesting she was like a Herodias or Jezebel putting the idea for the persecution in Nero's head.
But putting that aside I do enjoy a good Femme Fatale, but frankly Patricia did not make a good one in my opinion. I didn't find her very attractive and nothing in her performance is very compelling.
But Claudette Colbert was awesome, she was Sexy and conniving and vindictive but not without depth either, she does rather steal the show.
The cast lacks in direct parallels after that, surprisingly The Sign of The Cross has neither Peter nor Paul, but Titus and another old dude who's name escapes me. I don't know if that Titus was meant to be the one to whom Paul's Epistle is addressed. At any rate they play their role adequately.
In Quo Vadis, Paul's' role is smaller, already gone from Rome when the Fire happens. Truth is, based on the traditional sources it should be the opposite, The apocryphal Acts of Peter has Paul on his way to Rome when Peter is killed. They again play their role adequately, but not to compellingly, Peter's Crucifixion should have included some preaching from the cross, as is it feels kind of tacked on. And why are Peter and Paul so far apart in Age? I actually always assumed they'd be the same age.
No one else in SOTC really stands out after that. Quo Vadis has an again very undeveloped Acte (I find Acte's story fascinating, and don't really buy how it's handled here). And the Spanish slave girl is easily the most beautiful woman in the film, and gives a very passionate captivating performance but again doesn't feel to real to me, rather a little overdone.
The best character in Quo Vadis is probably Petronius, he's the character who's wise and noble despite not being a Christian at any point, "A heathen conceivably, but not an unenlightened one", providing the opposing voice when the discussion of scapegoating the Christians comes up. Leo Genn's performance is very good. Thing is, I would have given that role to Seneca, Seneca is in this film just is just another Roman senator, what makes him intriguing is lost, he was a Stoic philosopher, so Monotheism wouldn't have been unheard of to him. I could see him tolerating Christianity in a similar vain to Gamaliel in Acts.
Taking the comparison beyond just stacking up the characters. Like with Mercia SOTC in general shows more humanity and depth from the Christians. The Torture scene is very well done, and the kid being so frighted at martyrdom makes the film feel more real.
But Quo Vadis is longer, which especially for this genre I prefer. And with a bigger budget it's grander and has a more Epic/Theatrical/Operatic feel. The scenes of Nero and his entourage back at the palace during the fire, with Nero fiddling and then the Mob storming. And then the scene I mentioned earlier the debating whether to Scapegoat the Christians is very well done, my favorite part of the movie, I love how Petronius's whole argument is very well delivered.
So in general I'd have to say Quo Vadis comes off the more re-watchable film. Though a professional film critic would probably call SOTC the more well made film. I wish DeMille had gotten to remake more of his old Black and White films like he did The Ten Commandments.
However one thing I notice quite interestingly looking at my review is that The Sign of The Cross handles it's female characters better while Quo Vadis handled the male characters better. Not what you'd expect at all, an older film having the better written female leads then the younger one. And connecting to another observation of mine, I think DeMille was probably a more conservative person then the creative team behind Quo Vadis.