It depends on where you place the ads. For the 768banner it is good to place on the top while for the rectangular ad it can be placed in between content.
It depends on placement.If you place 728x90 properly it will get more clicks that's for sure.You can see most blogs use 728x90 ad unit above the fold and gets real real effect.
The only way to know which will work best for your particular site, niche, etc. is to test. Test, test, test.