I have never stayed at the Radisson. I have stayed at the Scanticon and offsite plenty of times.
For convenience purposes staying at the Scanticon, which is just above the convention floor is best. I have not walked the hallway to the Radisson more than five times in 17 years and don't intend to start.
For a vendor it's the nuts to be able to go to your room and get more product, make your own food, chill out, coordinate with friends etc...
If I were going as a visitor but not a player then I probably wouldn't mind staying offsite. As a player I might want to be close to where I am playing.
Very few venues these days offer onsite lodging. I find it to be quite helpful. I used to stay at the Homestead Suites down the road and it was definitely cheaper at $700 for the week. It's probably higher now.
I can't recall that we have ever had reservations at the Scanticon where there has not been some issue. My advice is to get NAMES of who you talked to AND the confirmation numbers AND to call back a day later to insure that the reservations were properly entered.
I doubt that we will be prepaying for our rooms either.