If both were 5.5 girth, I'd like the 8 incher.
If the 6 inch one was 6.5 girth, I'd take it over the 8 incher.
For me girth is more important. But if I can't have it thick, I'll have long thanks. But not too long. I have an inside depth of 8 inches. Any more than that and it hurts.
For me, the difference between 6 and 7 inches is huge. It's funny, you wouldn't think an inch would make that much of a difference but it does.