Based on your list, just gonna throw this out there...the F5 and the 21 both use 9v batteries..and both have knobs for adjustments, its good to standardize if you can, one less thing you know..
I think the F5 will make you a great land hunter pretty quick as it is easy to learn and operate, and you can quick dial the settings as the conditions change.. Those skills will transpose over to the 21. The 21 is a heavy machine for a reason, so if you are sweeping beaches or land with it, you will want to hip or chest mount the controls. I sweep the dry with the land unit, and shallow water too, but then switch over for deeper work with a buddies CZ 20. I like to go fast and hard up on the sand with a light quick machine, and then cool off and go slow out in the water with the 20, where its weight is not an issue, and its power is a benefit..I use a light scoop on the dry, and a heavy scoop out in the water. After a while, you will know depth and targets based on the tones. The TID is a helpful teaching tool and really great for relic or general purpose land hunting, but not necessary on the sand. Water hunting is tough if you try to go too fast, it forces a fellow to really slow down and relax. Hunting both mediums makes a person better at both I've noticed. Sand hunting both wet or dry will get a guy in shape in a hurry! Good Luck!
Mud
