We've stayed offsite before. The differences are this...
- Rental Car - If you stay on-site, you don't need a car (but you do need transportation to and from the airport). So you can save on the cost of the rental car.
- Parking- If you stay on-site, you don't need to pay for parking. On the upside, you may be able to find free parking nearby. Maybe. (We lucked out and never had to pay for parking. Don't ask me how that worked out.)
- Travel time - Origins is tough to make through with much sleep. If you stay off-site, consider the time it takes to get back and forth between the convention center and the hotel. I found it to be pretty wearing to be off-site.
- Cost - it is MUCH cheaper to stay offsite. We stayed at Fairfield Inn up near that university in Columbus and it was pretty cheap (although I just checked and it is NOT at all cheap now - costs as much as staying on-site!)
So for me...
Off-site - can be much cheaper if you have a car, find parking, and don't mind the driving.
On-site - having no car/parking offsets some of the cost, but the convenience of staying right there is pretty great.
Like everything, it all comes down to $$$.