I for one would suggest a little of both. I have about 1.5 hours of tunnel time which I dispersed all throughout my AFF training and subsequent A license trials. It helped a lot. Now that I am no longer nerar a tunnel I find I am missing it and it is a lot harder to progress skills in a 60 second free fall jump.
So, do a 5-15 minute block then do a coach jump or two and see how much it helped you. Make sure the coach jump relates somehow, if possible, to the tunnel practice.
This is how my coach recommended I do it and I think I benefited from it. Now, if only the midwest had a tunnel.
Don't forget, it also saves you money in the long run.
Just my .02 worth.