The DF is more user friendly, does not have to be 'tuned' as much, has a gain adjustment and has a single tone. The coil that comes with it is hard wired in, but is made for beach hunting and the stock configuration is very well balanced. Also, the headphones are waterproof.
The Infinium needs to be 'tuned' a bit more for the conditions, the stock coil ( DD ) is no where near ( in my opinion ) close to the 10x14 mono for beach hunting, so you will need to buy that coil. In addition, the stock configuration is brutal so you will end up buying a straight shaft for it. Also, you need to buy the water proof headphones since they don't come standard. It does have tone id in the sense that there are hi/lo signals for most gold, low conductors and iron, and the lo/hi for clad, iron and higher conductors...but being a PI hunter you will be digging all the signals anyway. You can also get an 8 inch mono coil for it for beach hunting. It also has ground tracking, being primarily a gold hunting unit, so black sand beaches and mineralized beaches are no problem. And it has a frequency adjustment so one can eliminate EMI or outside interference.
Saying all that, I have had great success with my Infinium even though I have only had it a few months. I think it is very sensitive to small gold, even with the 10x14 mono coil. I feel if you can run the Infinium at it's peak sensitivity level and deal with some of the chatter, it can sniff out beach gold and is one of the best beach PI's for the job. The secret is to get the unit just to the point of instability using the discrimination knob and threshold. Since the machine does not have any internal adjustment to cancel out salt water, you have to do it. Some beaches are less 'salty' than others, so you can tune the Infinium for optimal sensitivity to gold.
JC