While I am not familiar with IBM's Repair contractors, I would say that the onsite is always a better option, unless the laptop is something you rarely use. All of my laptops have onsite service.
#1. Faster service
#2. You know where your laptop is!
#3. You can explain & show the person the issue you are having, which usually results in a better outcome.
#4. You don't have to ship it back & deal with phone reps for hours trying to find out what happened to your laptop.
#5. you don't get your PC back from the depot, reformatted with the issue still there because they did not detail the problem well ewnough in your notes.
...However on the flip side
#1....It costs more thn depot repair
#2. Someone has to be there to meet the repair tech
To me the answer was on site repair. I like having the control & quick response, but it's all up to your personal preference.