In looking at maps for Verizon and Cingular it looks like most areas are covered well by both providers. There are pockets that have lesser coverage, but the major of the area looks good. You are going to find the prices for their rate plans are pretty similar and both providers will have low cost devices.
If you are interested in the "Push-to-Talk" feature (which you referred to as "Walkie-Talkie", Cingular has this feature for their users. -on to their voice plans. Keep in mind that you must have a voice plan in order to have the PTT add-on.
Both Cingular and Verizon have Family rate plans which allow you to pool your minutes (for all your employees) which would save on costs. Also, with either provider, calls between your employees will not count towards your minutes because they are considered "mobile-to-mobile" and the major of the rate plans include free, unlimited mobile to mobile calls.
Regarding customer service, Verizon was rated number 1 and Cingular was rated number 2. These types of surveys are typically conducted by companys hired by the providers, so I would not put too much stock in these. I would base my decision about customer service and reputation more on word of mouth, ie, people you know who have had good (or bad) experiences. This is a much better indicator.
The good news is that you can try each of the providers' service and if you are not happy with the coverage, you can cancel your service without penalty – with Verizon, you have 15 days, with Cingular you have 30 days. If you are not satisfied with the service, you can return the device and cancel your service without penalty. You only pay for the minutes you used, ie, pro-rated portion of the rate plan that you signed up for. No termination fees.
So, if I were you, I would pick either Verizon or Cingular and get one or two devices and try them for the trial period. If you are happy with the service, then I would get the rest of the devices. If you are unhappy, then I would try the other provider. Or better yet, get two devices from Verizon and two from Cingular (and sometimes even two from Sprint – they have a 14 day return policy) and see which ones work best for your employees. This way, you can see for yourself what works best for you and your employees and you will not have to guess or trust what the providers tell you.
One more FYI, all the providers will claim "least dropped calls, most reliable network, largest network", etc., but keep in mind that your location and the phone device also has a lot to do with your experience. Dropped calls typically relate more to the phone devices themselves.
One last thing. Verizon uses CDMA technology for their voice users and Cingular uses GSM (Sprint uses CDMA and T-Mobile uses GSM). GSM is the "global" standard, so people who travel overseas and want to use their phones, would want a GSM provider.