If you plan on swinging the ATX all day on land you better think of a ProSwing45 and do some intensive digging. I can not swing the ATX out of the water for more than 30 minutes while I can detect for 6 hours in the water.
I have used the 800 on land for a few hours, can't beat the information it gives you on the display and in the audio - plus all the user adjustments you can do on the audio, discrimination, etc. compared to the ATX. With all the positive features including the visual display I will not give up my ATX I have no comparison between the two in water or black sand areas, or the need to dig just about every target, except iron. I have yet to get my own 800 it is on the way from somewhere.
I also have the ATPro (and 12 other detectors), will I give up the ATPro for the 800 - NO - I need the sniper 4.5 inch coil to do some of my detecting. The 6" for the 800 would not allow enough room to swing the coil. Will one detect objects another will not? Of course - It all depends on your environment - soil, water (salt or fresh) local mineralization, local RFI sources, black sand, trash, etc. You see, every detector, coil, combination and area you plan to detect weigh heavily on what you should buy.
I suggest you write down your plans for a detector's use, the pros and cons including price, physical size if you plan to travel with it, retreiving tools, waders, wetsuit, waterproof headphone, etc. then you may make the best decision on which detector will be best for you.
I did not mention but if you have not read, all wireless connections will be lost when the detector's control box is submerged. Therefore a waterproof headset will be an additional cost.
I hope this helps.