This really depends on how you look at it, we prefer external for a few reasons which we will go over. But this does not mean internal battery mods are bad.
Internal battery mods are quite handy as they already come with the battery, so that already saves you on buying external batteries so YAY there. Internal devices also typically come in a smaller form factor which can be a huge plus if you are looking for a smaller device that can easily fit in any pocket. Downside is that once the battery has reached it's 500-600 charges it more than likely will be on its last leg. But this typically takes a year or so.
External battery devices are the more common and preferred for a few reason. One of them being is once the battery goes out after a year or two all you have to do is buy a new battery and you are good to go. External battery Mods also allow for "Hot Swapping" or in other words you can get a external charger (which does a better job at charging in half the time) and keep one battery charging while your other battery is still in use in your device. It also allows you to carry spare batteries with you if you are going to be gone for some time. External Batteries typically have a higher (mAh) or milliamp hour which means they will provide longer battery life during use.