Not that this is intended to be prescriptive, but I've started placing my DB/DP Dragonite in gyms due to sheer CP. Attacking has gotten so specialized, that if you have a dragon or two plus the usual suspects, you can get away with it. I don't personally know how your two dragons compare; however, I'd not want to use Steel Wing as an attacker, despite how good the charge move might be. But that's personal preference.
Another failed 100 % Drago Evo
So I evolved my lvl 12 Dratini 100 IV and got SW/O.
According to tripledex it performs much better than my DB/DP dragonite.
Should I now consider using my 3k+ dragonite with DB/DP as a defender for high gym placement and lvl up my SW/O dragonite as an attacker or just try another evolution?
Gym placement is very difficult here as a lower lvl 30 player without placing a dragon.
Answers
Let's be honest -- the defensive moveset isn't really as important as the CP. If someone's intent on taking down a gym, a better moveset will just cost them a bit of time & potions.
This site's ranking of movesets (https://pokemongo.gamepress.gg/pokemon/149) puts SW/O as a better defensive moveset than DB/DP; conversely, it thinks DB is better on offense than SW/O. YMMV, but if those relative rankings would suggest you power up the SW/O to gym standards and use the DB/DP for attacking. Or power both up for placement in gyms -- frankly, I never have problems using more common Pokemon (e.g. Eevolutions, Garys) to take down gyms.
I agree with both of you. My dragonite DB/DP feels kinda useless because I always attack with special attackers like exegg and so on. But still, I miss a fighting pokemon and Dragonite can be used very well against snorlax etc.
I really dont care anymore about the defensive movesets, my 2.9k snorlax is always number 9 or 10 in the gym. Its all about cp.. and SW/O is a stronger attacking moveset. Question is, taking the risk and evolve a third dragonite or lvl up the lvl 12 100 % SW/O
If you still need gym defenders, just power it up instead of "wasting" more candies on another Dratini...
As mentioned, movesets are overated in gym defense IMO and someone who wants to take your gym simply WILL...
My crap move Blisseys are almost always in gyms and a stack of 3 or 4 Blisseys on the bottom of the gym is the biggest time/potion deterrent for most people...
A 100% nite means power up for gym defense now, at least until the gym rework...
I'm still waiting for a high IV Dratini (hatched 2 so far) and zilch.
It'll be nice when I can wake up and press collect coins every 21 hours.
Defenders are based on CP mainly in this meta, with the primary move as the second determining factor, and the charge move as the third.
SW is still a good defender move, not as great as DT in most cases but very annoying against nite counters. What you have is a strong defender that's only slightly clunky when used on offense.