Depends on the job and what you're doing with it.
According to Tractor Data your tractor is 59.8" wide (minimum), so a 6' rake will just barely cover the wheel tracks. If you angle the rake then your raking less than the width of the tractor. If that's important or not is up to you.
If you're raking miles of gravel road an extra 6" on each side may make for a quicker job. If your raking in VERY tight spaces (between trees for example) a 6' may be the right choice. Again, your call.
Then consider - what are you going to do with it once this "job" is done. More jobs? Personal use around your property? Etc.
If I had your tractor I would want the 7'. JMHO