UPD can also map to printers on other Windows computers that you set up as shared devices in a peer-to-peer Windows network -- something Mobile Express can't do. On the other hand, Mobile Express allows you to add subnets to a location -- something that can come in handy in offices where wireless and wired LANs are in use. UPD doesn't let the user add subnets to locations, but you can access any printer by its name or IP address and print to it. Administrators can also create managed printer lists that can be made available to UPD users, regardless of subnet.
While the installed base of printers in most offices includes the HP brand, the reality is that most offices have a mix of brands. In those environments, a tool that's limited to managing only one brand of printers doesn't solve the larger problem for administrators or end users.
For my purposes, neither tool had a clear advantage over the other. The discovery tools in UPD and Mobile Express are fine as far as they go, but based on my experience, you'll need to be prepared to find printers by their IP address if they don't show up on the list.
UPD will work with any printer -- as long as it's an HP. It might work for you if you have networked HP printers in all of your office locations, but it won't help you quickly connect to nonnetworked printers or non-HP printers that you might encounter while traveling.
Mobile Express will work with any brand of printer -- as long as it supports PostScript. That leaves a lot of printers out in the cold, especially low-end models that may be direct-attached in home offices. While most corporate offices have networked printers these days, not all support PostScript. And the low-end, all-in-one ink-jet printers typically found in the home and satellite offices of many mobile users often don't support PostScript either.
The best fit for these tools may be folks who travel between offices within the same company. In each location, users are more likely to find a variety of networked print devices, at least one of which might be HP-branded or PostScript-compatible.
In my case, I could make either work as my single-click printing solution for all three locations. But not without a few changes. With UPD, I'd need to buy at least one print server; with Mobile Express, I'd need to buy another PostScript printer for one location.
I've decided to keep both tools on hand. They might come in handy for situations where I need to quickly access a printer but don't have the right driver installed -- a sort of skeleton key for unlocking access to printers anywhere. For everyday use, however, I'll stick with the drivers I have -- and hope for Xerox to come out with a version of Mobile Express that supports PCL.