It will really depend how much space you have on your RDA deck and how skilled you are to shape your coil(s) With small gauge (ie. large diameter) wire like that, the resistance will tend to be very low, so you need more length to compensate (hence the space constraint issue, as most often you don't want the coil to touch the atty walls, even with glass but obviously not at all when it's metallic and conductive.
Just never ever make super low ohms builds on a mech mod. You really need some kind of electronic protection to prevent your cell from exceeding its CDR, so VW or TC only. It's not uncommon to end up around 0.2 or less with that large of a wire and little space, and it can be hard for the DC/DC converter, as well as creating other complicated issues with TC (resistance changes are so small that you start getting resolution/quantization issues with the ADC used to measure said resistance, the ohm-meter being done in software on the mod microcontroller)
You can experiment with dual coils builds too, but in my experience they add inertia and latency to the system for some reasons. It's also harder to shape two "perfect" coils rather than a single one.
So if you are not too confident, go with 22awg instead of 20awg. I would reserve 18-20 for more advanced users.
Also have a look at SS430, it has little (if any) Nickel content, so it's a good path to explore for those with Ni sensitivity (or just to stay even more on the safe side, since we have practically no study proving we are not inhaling metal particles over time, apart from that old biased e-cig study where they over-cooked the thing so much they found Ni particles everywhere in the tank afterwards...) Small coils do seem to degrade over time, so it's always an open question, nothing being created, nothing being lost, where does the metal go if not in our lungs?
That being said, the larger the wire the less glow and with 18-22 you should get a build that never ever glows. The steel will turn from silver to yellowish at most but will not go through all the rainbow colors... or well unless you dry fire the thing like crazy. A good strong coil should last forever if not over stressed, or at least that's my belief (and our coils are not immersed in corrosive liquids and don't get caked with crud stuff like ecigs, they need to change theirs because they are small and they get all crusty with overcooked stuff and taste degrades, we don't have these issues)
But we do face oxidation, and the more it glows in an oxygen rich environment, the more it will be attacked. I think we all agree now that the least glow the better, at least until proven otherwise, just to be safe.