Nothing wrong with using the octo as your primary regulator, as they - the octo & primary - deliver air just as efficiently.
However, do bear in mind that in emergency scenarios - say, your dive buddy runs out of air at 30m. And panics. The first thing your buddy will probably do is look for a source of air and under duress, will reach out at the brightest thing they see.
And there you are, looking at your buddy while happily breathing with your yellow "primary" octopus and wondering why his/her eyes are so wide.
Of course, as you log more dives and gain more experience, the calmer you - and hopefully your buddy - will be underwater. Hence, putting you both in better positions to control the situation
On a different note, primary regulators normally have better build quality (hence the difference in price) as they tend to get utilised more in a dive, with the octo being given a role reserved for brief/emergency situations, like buddy breathing - whereby should it happen, you would normally terminate the dive and begin your ascend/safety stop.
So i guess it boils down to your level of comfort with your equipment - in terms of performance, durability etc (not to mention, hoping that your buddy is colour blind too). Personally tho, I would go for the primary/octo combo.
On the hindsight, try not to mix the brands and models of your primary and octo combo when purchasing, as some may have different PSI configurations.
