I actually like both romances, but I prefer Zev's (duh, that's why I'm in here more often.

I usually only do the Alistair romance with a human noble. I'm going to do the run off to help the GW ending at some point with someone that is not a human noble at some point (I have an elf mage character made, but is still waiting to finish her harrowing. elf mage is the only elf background that *I* could play that would romance a human ) and a dwarf made as well, just to see what it looks like) (I just can't play the mistress. No female warden I've made could possibly make that choice (I guess it's my own feelings that play into that) and I'd like to do a political marrage with Alistair at least once, but I have a hard time making Zev be the boy on the side for the same reasons)
almost everyone else is with Zev (I have a few men for Mori and a male and female for Leli as well)
I find Zev's romance the most appealing to me. I do find Alistair a first love romance, but sometimes first loves do actually make it (my own mom and dad is an example and my ex bf's parents knew each other only a month before they got married and they are still together too.) so I find it a really sweet romance and I do believe he really does fall for the warden, but as they say sometimes love just isn't enough. it's bittersweet.
Zev's romance on the other hand can start off as totally causal, but there is something about that damn sexy elf that keeps bringing me back. All my characters (even the one's that do not romance him can agree with his assessment that "some people just need assassinated" so they don't find him or his beliefs completely alien to them. It's adorable to see this man that is supposed to be an amazing seducer stumble over the right words to say to the warden he is interested in. as bad as the poetry reading is it's just so cute to see him try.
I don't believe he was really in love with Renna either. I think at the time he THOUGHT he was. there was a time when I thought I was in love with someone and realized a bit too late that I was in love with the IDEA of him, or the version of him I had in my mind and not the real guy. (the real guy was a verbally and emotionally abusive turd that cheated on me twice) while my mind saw a tall, dark, and handsome man with a sexy accent that took me to new places and helped me experience new things that I'd never seen or done before. I see what he saw in Reena the same way. It was the idea Renna and not the real person inside that he fell for. I also think he got scared of the feelings he had and found a convenient way to get away from it. sure it was a cold thing to do, but Zev's eyes didn't really open until after his confrontation with the Crow master afterward.