E-M5.3 is smaller and lighter, and has a few differences in specs, but has the same sensor as the E-M1.2. The E-M1.2 has more custom buttons and more battery life, but it is also all around larger. If you're looking for something to pair with primes and other light lenses, I'd say the E-M5. If you are shooting larger lenses that have a lot of OIS soaking up battery, the E-M1.2. Keep in mind a grip can be added to the E-M5 for better handling with large lenses, but it won't add to battery life. That said, the E-M5 batteries are tiny, so a few extras are not going to be hard to pack.