Yes, the engine is important of course, but let's not pretend that a heavier hybrid is ever going to ride as quickly as a cyclocross bike. IF speed is a consideration, I would opt for the cyclocross, but IF it isn't, then I would modify the hybrid. A lot of it is personal preference and depends on how fast you like to ride. Some people have no interest in going "fast," while others have no interest in going "slow." I come from a racing background and riding for speed is hardwired into my system, so it's often my preference or at least a consideration.

For example I used to commute to a job that was about 20 miles away. I rode my racing bike at a fast/brisk pace so that I was getting a good workout and still getting to/from work in about an hour. There were showers and a place to stow my stuff, so I treated it like a training ride. More recently, when I was in grad school, I had a 5 mile commute, but nowhere to change or take a shower. I used a hybrid and kept the pace down. It was enjoyable, I got a little exercise, and I wasn't a sweaty mess at school.

So, what I'm trying to say, is that along with the types of rides you are considering, you should also plan on how you plan to ride them.

Personally, I would probably work towards having both types of bikes as they would give you a LOT of flexibility for different types of riding. With a cyclocross bike, you can set it up with rear racks and a handlebar bag and do some light touring and then take that stuff off and put on narrower tires and have a pretty decent road bike if that is attractive. Since you already have the hybrid and the cost is essentially sunk, so I would keep it for urban riding, grocery stores, short rides, just plain fun, etc. But again, that's just me, and I'm not saying what you should do.