The PowerBook G4 (TiBook) will give you noticably better performance.
I don't have specific benchmarcks for particular models, but here's a quick discussion of the various factors:
Level 3 (L3) Cache: When processor speeds are relatively close, the presence of the L3 cache will usually have the biggest effect on performance. VPC often translates a block of x86 code which is then called repeatedly. An L3 cache can often hold an entire block, which avoids the slow loading of the block from main RAM accross the system bus.
Of Apple's currently shipping models, the 933 MHz Power Mac G4, Dual 1 GHz Power Mac G4, and PowerBook G4s have L3 caches. The 800 MHz Power Mac G4, iMacs, iBooks, and eMacs do not.
Processor Speed: When all else is equal, more is better. VPC is compute intensive, so a 20% faster processor will usually feel 20% faster.
Bus Speed: A faster bus will usually yield a noticable speed increase. VPC often has to transfer a lot of data to and from RAM over the system bus.
Dual Processors: A minor but noticable speed increase. Because VPC emulates a single Pentium, most of Virtual PC must run on a single processor. However, video updates can be offloaded to a second processor, freeing up the primary processor for an emulation speed increase around 15%.
AltiVec: A minor speed increase on single-processor G4s, probably only noticable with a stopwatch. Pentium emulation does not take advantage of AltiVec because there is no AltiVec equivalent in the Pentium instruction set. However, video updates take advantage of AltiVec if these have not already been offloaded to a second processor.
Hard Drive Speed: Performance depends on what Windows is doing. Disk intensive operations like booting Windows will be faster on desktop machines.
