Having tested this extensively when I worked for an ISP who wanted to offer a solution based along these lines, the basic version is, all the options are rubbish...
round robin doesn't work properly, breaks some traffic, causes horrific jitter and is generally as rubbish as you'd expect it to be...
line bonding was a nightmare and I quite understand why most ISPs can't be bothered offering it. I won't go into the specific issues as nobody will understand I suspect but suffice to say it was worth the effort by a long way
The only option which even half worked was load balancing between two tunnel based VPNs, one over each connection, terminating on a router in the datacenter and piped out from there. I had that setup at home for a while before I moved but commercially it was too expensive to be worth considering...and jitter was still an issue...