If you are needing to back up that much data for archiving, I would recommend purchasing an external Blu-ray drive and double layer (50GB) discs.
Regarding durability, optical discs are not susceptible to magnetic damage like hard disks are. This is a big advantage IMO. They are susceptible to "bit rot" where the disc may "age" over time and possibly become unreadable. If you use quality media, however, this should not be a problem. The scratches, heat, etc., should not be an issue if the discs are cared for as they are supposed to be - stored in a jewel case and placed in a safe location.
I personally will not back up any of my data to any "cloud" backup service. It is just a matter of time until these services become compromised, and once your data is out there, it is out there. There is no getting it back or removing it. Photos and music may not be very private, but when it comes to other files and data such as passwords, financial information , business data and other sensitive data - I have not and will not allow this to be stored on someone else's servers that I do not have control over. Dropbox has already had a major security issue reported in the past year where you could log into any account with any password you chose (you could type anything in the password field and it would accept it). This is one example. Who knows how many other times this may occur, and even it it will be reported. Storage is cheap - 3TB drives are inexpensive, and if your data has value the peace of mind is well worth the cost of an external drive.