Having worked with both and priced out both, IPStanding is correct, the S8300 will cost you more. Also to get some of the features in the S8300 also has a price tag.
If you're looking to go full IP, I would look more towards the S8300 (personally) than an IP Office, only with some issues I've ran into in the past with IP phones on the IP Office. However, as I'm sure you'll hear from a few others here, they've had great success with SCN and multiple IP phones.
I think the S8300 was designed more as a system to be a LSP off of a larger system (ie S8500/S8700) and they just threw in the ability for it to stand alone as well for another option.
Lastly, if you look in the Avaya Definity forms, I think you will find less "issues" with the S8300 than what people have had with the IP Office (though they're lessening over time).
Again, this is just a personal view.