You first need to consider if you even have the budget for internal nails, these nails alone are the price of a down payment on a house. Guichet, for example, costs roughly 60-70 thousand pounds to have 2 Guichet nails on both tibia and femur. Life changing money, especially if you get into debt over it (highly unrecommended)
Internal nails will give you the benefits of not having much scarring present after lengthening. Plus the lengthening process is generally considered less painful and risky. The reason for the lower risk is that pin-site infections are non-existent with internal methods.
Mostly, the reason internal nails are favoured is because they have a lower chance of infection (deep bone infection is rare). They offer superior recovery and aesthetically pleasing results. They are more practical for purposes of moving around whilst lengthening. Plus having a massive frame attached to your leg may make you feel emotionally tormented that you are damaging yourself based on social constructs etc.
The overall process is usually shorter with nails as you can go home earlier and lengthen.
It all depends on what you can afford, there is a millionaire app developer on here who was able to go straight to Paley and have the best technology through Precise II .. however that may not be in your budget. Basically, get the best you can afford. Always save for the best doctor you can go with. But leave yourself a considerable amount of money for complications and Physio etc.
Knee damage is present in internal methods as the Patella tendon is split whilst tibial lengthening (and in some cases femoral lengthening) .. it is not guaranteed you will have knee pain. But to avoid it, you could get internals in your femurs and externals in your tibias. If I'm honest though I don't want to scar my lower leg too much.
Just remember that there are so many other things that you need to consider when lengthening, you are going to be basically crippled for roughly a year. That in itself can have it's toll.