Welcome JC! You're comparison is apples-to-Buicks... but I'll try to give you some examples of why.... The prebuilt coils have been known to have "packed" cotton, which may be too tight, which slows wicking-action. Many people have said they needed to loosen the cotton a bit to improve wicking in the pre-built coils. Use a toothpick or push-pin to loosen it up a bit if that is the case. When you are building on the RBA deck, you are most likely "fanning" the cotton, which improves wicking greatly over the pre-builts. As for power, 0.5 Ohms @ 40W is 4.5V and nearly 9A of power. The 0.7 Ohm coil @ 40W is 5.3V, but only 7.6A of power! Is 40W too much for either of these coils? That depends on the juice and your preferences, but you can see the Amp-draw and Voltage differences. Personally, I have a 0.8 Ohm coil I use often, but rarely push it beyond 25W of power... then again, I don't like my vape HOT, nor does my juice