We don't make specific device recommendations as your choice ultimately depends on your overall needs. There are a wide variety of devices at various quality and price points.
Generally speaking we recommend that you avoid generic bottom of the range devices, as these are usually cheap for a reason, specifically poor GPS, display and processing components. Mid range devices from name brand manufacturers such as Sony, Samsung, Apple, Motorola, HTC, LG, Asus etc. are usually reliable choices.
A suggested approach is to trial preferred phone/tablet devices for a few days in the field. This way, if the device is not liked by you or your staff, you have only bought a few and can make a change to another device. If the trial works well and your staff like the device, you know its a good fit and you can purchase more. It's usually more cost effective to buy in bulk as you can negotiate a discount with your supplier.
- Generally we believe that 7 inch tablets are the best option for pure app usage as you'll have more screen space in the tablet form factor. The the 7' are still quite light-weight and easy to carry compared to 10' tablets.
- If you need your staff to be able to make calls, enter data, etc. a 4.5' or larger phone would be ideal. Larger screens make app use easier for people with 'fat fingers' :).