We don't make any specific device recommendations since it really depends on what your needs are.
There are a wide variety of devices at various price and quality points.
Generally speaking we recommend you avoid generic bottom of the range devices, since these are usually very cheap for a reason - namely 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 a couple of your preferred phone/tablet devices for a few days in the field first.
If the trial works out well and your staff like the devices, then you know its a good fit and you should aim to standardise on that device if possible.
Generally we believe that 7" tablets are the best option for pure app usage since you have more screen space in the tablet form factor, while 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 etc, then a 4.5" or larger phone would be ideal.
Larger screens make app use that little bit easier for people with "fat fingers" :)