Cron would be easier on your system. do you really wanna run an sql statement every time a person loads a page just to see if a certain amount of time has passed? That would be less productive than a single cron job ever so often.
I'd prefer not to have cron because every place i knew had a cron, and created similar "turns", would lag down everytime the turn would take place. Not very important when one has a small amount of users, but becomes kind of a drag when the community becomes big.
Also, since i still have to make around 2-3 sql query per page, so, having some code checking the time isnt that big of a deal.
But... i'll try both ways

Then i'll check how it goes with one and hte other

Thanks for the input.