Since most veterinary packages, word processors, spread sheets and other programs are written for IBM compatible computers, you maintain the most flexibility if you insist on an IBM compatible machine. Buying IBM compatible hardware means that you are not locked into a single vendor of hardware or software. The widespread availability of IBM compatibles means that hardware upgrades and software packages for these systems are much less expensive than for less popular computer types. There are hundreds of brands of IBM compatible computers.
Quality varies somewhat between brands.
What is most important is the warranty and service provided with the machine. Two brands we have had good results with are HP and Dell. One brand we have not had good results with is Gateway. Your own practice size, methods and future plans should be considered in this decision. However, larger practices will probably want to have several workstations available so that a number of people can be working simultaneously.