In a match against Karlovic you can play well and lose or play bad and win. It's up to a few points.
If the match turns into a TB fest, I think Tomic has the edge, because he is (a bit) more athletic and being 196cm he has the huge range to put some Karlovic returnable serves in court. But I'm not sure Tomic will be able to concentrate and hold the serve for the whole match or lose just once like in Bogota, after all, it's R1 and not a final. For Karlovic doesn't matter much the place really, he normally can hold serve in basically any tournament.
Still going with Tomic, but this could go either way.