There are differences for sure, weight being the biggest factor (big heavy dudes wil have a harder time climbing against stick figures) but in terms of your own personal performance, your success will almost entirely depend on your ability to generate the required power over the time of the ride. If you could train at that similar power on flat lands, you will be equally prepared. It's not like you suddenly lose 10+% of your power because you suddenly start climbing.
I'm a believer of being able to train flats to ride big climbs. I have a local loop with 6000 feet of climbing in 50 miles right out my front door, and I'm plenty well prepared for them by just doing aggresive trainer rides of adequate length. The main thing is to make sure you're doing enough time on the flats - since you might be going 8mph up a long climb, you'll have to adjust for that duration on a flat effort so you don't get rudely awakened when you realize that a 50 mile hilly ride can take an hour longer than a flat one.
Another consideration for beginners or weaker riders is of course, if the % grade of the hill is too steep for them to generate enough power to keep cadence reasonable, which often happens over 12+% grades even on a triple. Even then though, you'll be just a good hammering it on a flat road or trainer at similar wattage of the climb, rather than dismounting and walking the hill if you're trying to ride it repeatedly without being ready for it.