A: If we were comedians (or smart alecks), we would say, “one.” But since we’re neither of those things, we’ll say this: in some ways, the two units are quite similar. For example, both use the same T4 ELOP optical module. The controls are very similar, and both function and are used in the same way.
Among the differences between the two are the physical format (3-rack space LA-2A versus half-rack-wide LA-3A), and, most important, the LA-2A is a tube-based unit while the LA-3A uses all solid-state circuitry. This last fact results in a big difference in the sound and response of the units, and determines the applications each is commonly used for.
The LA-3A has faster attack and release times for larger transients, which makes it suitable for drums, overheads, and room mics, as well as other signals that have a big attack, such as piano. It’s popular for electric guitar because of the way it emphasizes midrange frequencies.
The LA-2A tends to not be used for these types of signals. Rather, it’s commonly applied to vocals, bass, and many other signals that don’t have large transients or require fast attack and release times.
Having said all that, the above is not a list of rules. An LA-3A can certainly be applied to vocals, if it provides the sound you’re after. Likewise, it’s not unheard of to use an LA-2A on drums. As with just about everything sound-related, what your ears tell use is best is what is correct.
By the way, both the LA-2A and LA-3A have been very accurately modeled as plug-ins for Universal Audio’s UAD cards.