Establish a list of comparison criteria. To find the individual bike that is best suited to your needs, write down the three most important characteristics. If you want a cheap back-up stunt bike for practice rides, then price would be your primary comparison criteria. If you want either bike as your primary machine for stunt contests, then condition and service history are important criteria. XR 70 models are likely to be older than the CRF 70 versions so if you're looking for a new or nearly bike, then focus on the latter.
Eliminate any irrelevant criteria from the comparison. By establishing the most important comparison criteria, you should have an idea of characteristics that are less important. Search used bike ads online and review the summary of each bike. People are likely to list the best features of their bike first. The paint color or the condition of tires may be unimportant so avoid paying for these characteristics by eliminating any bikes that advertise these features prominently.
Make a list of secondary criteria. Factors such as age or the bike or location of seller may be less important, but in the event of a deadlock you can use these criteria to pick the right bike.
Devise a simple ranking system. Draw three columns on a piece of paper and head the columns with your three most important criteria placing the most important on the left. Rank each bike that you are reviewing out of 20 for your most important criteria, out of 10 for the second most important criteria and out of five for the third. This method gives an accurate reflection of each bike's suitability while taking emotion out of the process.
Compare the same characteristics of each bike. By reviewing the various available versions of both models, you'll get an idea of which model is more suited to you. If you're seeking a more modern bike and are prepared to pay extra, then the CRF 70 may be more suitable. If you want a cheaper bike to practice on and you aren't concerned about advanced features, the XR 70 is more suitable. To get a fair comparison review and score both models separately. Once you've done this, add up the scores and select the individual bike that best meets your personal criteria.