That's *really* hard...But I'll try to pick one. They're both gorgeous in their very unique ways. I like Zvonimira a little better... I really have a thing for Slavic names!
SieglindeThough it sounds totally outdated, but it's still better than Zvonimira. It's pronounced zeek-LIN-də, by the way. Zvonimira just sounds harsh and looks ugly, too.
Sieglinde. But I'm pronouncing it "SEEG-lind". Zvonimira is a tad TOO exotic for my tastes...and I keep forgetting about the first i and typing Zvonmira. Sieglinde is a bit more...together.