Well, ArcEm predates VRPC by a long time - the original copyright statement gives 1995 as the earliest date. The difference is that VRPC uses various proprietary (but not particularly novel) techniques to speed up the emulated system.
As to whether you get a fast emulated system, the most significant question is this: why would you want one? Sure, there were some nice applications available for RISC OS in their day, but the people with the expertise to make a nice, open, fast emulator/virtualiser are more likely to be spending their time/effort on much more useful stuff than extending the lifespan of old, proprietary software. Here, "useful stuff" typically means "applications for contemporary platforms", as seen with Jonathan Marten's DrawView developments, for example.