Size matters when it comes to experiments. The top one is applicable to small samples such as sinoatrial node, atrioventricular node, atria and ventricles isolated from mouse, rat, rabbit, guinea pig, dog and goat. The bottom one is not only for the in vivo study area where the probe is difficult to approach, e.g. posterior of ventricles.; but also for the studies on the ventricular epicardium from the larger animals such as sheep, pig and dog.