Either way, you’re limited the amount of space they give you. With Dropbox, if you have the free account you’re likely to run out of space with the first backup (presuming you have any other files on there). Gmail is only about 15GB so eventually you’d run out there as well.
You should probably set up an Amazon S3 account and upgrade to BackWPup Pro to enable differential backups so you don’t have to upload 1GB+ files every time you go to back up your blog. While AzS3 and upgrading to BWPUP Pro is going to cost you some bones (you pay for what you use) it’s surely the best option I think.
For me, personally, I just keep the backups on my shared hosting server (HostGator). While it’s kinda dirty because I also host my blogs there (ideally they’d be separate in case one filesystem fails) I’ve never had any problem with files disappearing. I download them from there to my local backup drive just in case. The clients whose blogs I host aren’t terribly concerned with backups because their sites aren’t their livelihood, so I do it out of courtesy. If they ever want AzS3 backups (even better: Amazon Glacier – hope this happens! that would make it easier for me to sell it to them, not being as expensive) I’d be happy to do that for them.
