I think the first thing you must consider is what programs will you be running. If these are programs that only run in Windows with no Mac version then you have two options. Before we get to the options there is also some businesses that used web based intranets and a lot of these use active X, which of course is a Windows only feature too.
What are the options? A Mac running Boot Camp so that you can install Windows to run those programs that won't run on your Mac. If you do have Mac versions of these programs consider the expense of having to switch. This will allow you to continue to have Windows PC, but also have Mac side to start learning and/or slowly move over too. The other option is to just stay with a Windows based PC so that you can use all your existing software so you would have no additional expense.
In consider PC's I'd recommend a Lenovo ThinkPad, these are some really nice running machines with wide open front side bus with fast memory. They come with really good components, decent size hard drives and even fingerprint readers if you want that extra layer of security. Most of them have a matte display so you have no issues with glare. HP also have a really nice business line of computers, both offer 3 years on site warranty's on their business machines.
If you decide to go Mac,I strongly suggest you get a lowend model and upgrade it yourself as you can save $300 to $400 depending on the model that you get. I purchased the 13'3" MacBook Pro. It is a rockin machine, but I do have to use Windows sometimes. This is not a big deal and it works great. You can even do VMWare or Parrelles and never have to exit out of the Mac OS. However, in all honestly for me I found that Boot Camp works best because for the most part you Mac because a Windows PC when you boot into Windows fully.
Lots to consider, but having done both for a while now. I really think it all boils down to how expensive it would be to convert over your software if it is available and then go from there. I bought my Mac's more for personal reasons than business ones. They do the job no doubt about it, but there are a few things that I simply can't find a replacement for under the Mac OS. So, given that scenario if I was buying for work or a work based machine, I'd go with a PC. In fact I still use my Lenovo T400 Thinkpad each and every day during the work week. I've not been willing to give it up to my MacBook Pro yet and I love my MacBook Pro. Personally I think Desktop's are so yesterday! I have no desire to use one anymore for work!