We are a Dell shop here but also have a few Macs and IBM/Lenovos, too. Desktops were Optiplex but the last couple of years we have been getting the XPS models for the most part. Optiplex model is not flawless but it is a reliable workhorse. For a short while we also got the Vostro model, not sure if they still make 'em, but
not recommended!
XPS models have been decent for the most part but their ever-changing video card made deployments a bit annoying. Also XPS 8300 model came with pretty good ATI/AMD graphics cards initially but they quickly switched to inferior models and eventually to a very obscure, unmarked model which auto detection tools couldn't even identify. They came with Win 8 and if you wanted to installed a 32-bit version of Windows 7, you will have tough time locating the drivers as Dell doesn't support it.
The late 8700 models also switched to Nvidia 630 graphics which made things much better.
Also bunch of XPS 8300 NIC started failing anywhere from a month to a year, that was annoying and required motherboard replacement since they were under warranty. XPS 8500 and 8700 models, despite the default low-end graphics card they come with, have been good.
Still, I would also recommend a 1/2-year and preferably a 3-year warranty for those who hold onto their PCs for 3+ years.
We also got a batch of Optiplex 7010 machines, not as slick looking as XPS but definitely a solid machine with the same exact internals.
For those looking to place their monitor on top of the PC, Optiplex 3010/7010 or a similar model would be the way to go as they have a horizontal layout (DVD drives, connections, slim profile, etc.), too.
For those who have their PC stand on its own, XPS might be a better choice.
Latitude is what I normally work on and a few MacBooks, iPads and Lenovo laptops/netbooks (X120e and X1 Carbon). X120e was meh, but I was really impressed with Lenovo X1 Carbon.
Even though I like the Latitude models, for anyone looking for a slim, light weight laptop/Netbook, I'd say, take a look at Lenovo X1 Carbon, too.
One thing that annoys me the most with all the brands I worked on, was, even after putting the Service Tag #, you still get multiple device drivers for most of the devices! I could see this turning into a daunting process for Novice users who are skilled enough to install/re-image their own OS.