If I were you, I wouldn't worry about boot camp. It's just a couple months and none of them are "hard" in the sense that I think you mean. Yes, they will push you to your limits in Marine Corps boot camp but they're there to make you into a Marine, not to make you fail. So while things will be "hard" while you're in boot camp, it's only temporary and it will just make you a better person. The most important thing to think about is your life after boot camp. If you don't want to go to war, you might not want to join the Marines. Not all Marines will see combat, but many (most?) of them will. On the other hand, the USCG does have a fairly high probability of seeing violent conflict in certain jobs and locations. In certain areas, you'll be spending a lot of time attempting to stop drug cartels.
My point is, just think about what you want to do AFTER boot camp. Boot camp is such a small part of your military career that worrying about its difficulty is just not worth it. Think of what you want to do, do your research, and go for it!