Speed depends on the cards and the reader (or writer in the camera's case). The camera's ability to write to them any faster won't have any effect on the quality of the images, just how fast you can make and store them.
From a reliability standpoint, I'd make some changes to what you're doing.
SD and CF cards use flash memory to store what you write to them. Flash doesn't last forever, and eventually it will start having unrecoverable problems. Exactly when that happens depends on a lot of variables, the biggest of which is how much data has been written to it. For the sake of simplicity, let's say your cards last a year of shooting before they start experiencing errors.
What you don't want is to pull your SD card out of the camera after a year of shooting, find that it's failed and then discover the same thing when you pull out the CF card you're using for backup. That would be bad.
You can avoid getting into this situation by making sure both cards don't wear out at the same time. The easy way to do this is to stagger replacement of your cards by half their lifetime or, if you don't know that, some interval large enough that you won't lose pictures. Installing a new backup card six months after your primary will, all other things being equal, offset its failure date by that much. When your primary experiences a wear-related failure, the backup will have six months' fewer writes on it and will be unlikely to fail for the same reason.
If you do your purchasing on a staggered schedule as well, you're less likely to get two cards from the same batch, which protects you against bad batches and won't lose you any data if you replace one of the cards with one that's bad out of the box or turns out to be a counterfeit that doesn't last nearly as long as the genuine article.