In terms of preparing you, they will be extremely close and many questions will be exactly the same. As you may know, the R version was from the paper administration of the test. The CBT version, as the name implies, was slightly modified for computer administration. Another difference is that the R versions are typically longer as the MCAT was a longer test back then. If you already have one version and are simply looking for good preparation, purchasing the other of the same test probably wouldn't be worth it. They'll both be good for that purpose. The most important caveat is that the CBT versions will be more accurate for gauging your score.
