So I'd start off with this math. Standard attacking build is something like 10 EPA and 8 DSPA.
So the orc starts off with: 77 OPA and 32 DPA. The avian 60 OPA and 32 DPA. Lets pit them against eachother in a 1 on 1 battle, which doesn't take into account chaining.
Lets assume that both can 2x eachother but can't 3x eachother to start. Orc is close to opening with 3x'ing, so that might blow this comparison out of the water, but in reality it likely doesn't since I would imagine avian with its cheaper troops (and being cleric) and more flexibility with building in EoW probably has slightly more military than the orc.
So go in Excel:
Column 1: Orc offense = 77 Column 2 Orc defense = 32. Column 3 Avian offense = 60, Column 4 Avian defense = 32. Column 5 is Column 1/4 = 2.41. Column 6 is 3/2 = 1.88. So the orc can 2.41x the avian and the avian can 1.88x the orc. In reality people run TGs more than forts so both of these numbers will be higher, and in reality its much more likely orc is like 9 EPA and 7.5 DSPA compared to avian 9.5 EPA and 8, etc. But lets go through the exercise anyways.
Then make these formulas below the numbers above, assumes 5% troop kills for non-cleric and 2.5% troop kills for cleric and both sides 2x'ing
Column 1 Row 2 = 77*.95 = 73.15 orc offense left after first wave
Column 2 Row 2 = 32*.95^2 = 28.88 defense left after first wave getting hit twice
Column 3 Row 2 = 60*.975= 58.5 avian offense left after first wave
Column 3 Row 2 = 32*(1-.025*1.15)^2 (to account for orc kill bonus) = 30.19 defense left after getting hit twice.
Then after the first wave the ratios went from 2.41 and 1.88 to 2.42 and 2.03. Drag it down and you'll see that by the 4th wave, the avian outpaces the orc, where the avian can hit the orc 2.55 to the orcs 2.48. It only goes down after that.
The standard war kd setup will have 10 or more support TM provinces that aren't hitting. So in the scenario where the Avian kingdom initially hostiles the orc kingdom, the Avian is hitting the orc all home to start, killing offense. If the avian can only 1x, that turns the tide so that it only takes 3 waves for the avian to outpace the orc. If the avian can 2x on the orc turtled, that turns the tide after just 2 rounds of hits.
In the event the orc kingdom waves the avian kingdom, its clear the avian kingdom can just drag out hostile for 3-4 waves and basically enter war already having a clear military advantage, despite the orc being +10% in war and having 1 point higher elite. That's a good situation to be in. So the avian can open on the orc and do alright, but if the orc gives the button to the avian, it's in pretty big trouble. Orc will run some hospitals, but as you will see below, 15% hosps and 64% BE is only about -25% savings.
So militarily I like the avian from a numbers standpoint, I think its easier with cleric to fight back to back wars as your elite count stays higher, and it seems as if an orc kingdom waved my avian cleric kingdom, the avian kingdom would win simply by dragging out hostile.
So lets compare the rest of the bonuses, namely -30% attack time for avian and +30% gains and 1 general for orc.
A standard orc war build will be something like 10% guilds 4% towers 15% rax 15% tgs 15% gs 14% WTs 15% hospitals 1% dungeons 10% farms. This will give the orc 11.26 hour war attack time vs an avian on no barracks less than 9 hour attack time. Take the 11.26/1.3 for gains and you get the orc on roughly 8.66 equivalent attack time. So orc has the benefit of one extra general.
Is that extra general worth the 30% buildspace avian saves by not having 15% rax and hosps? I'd say no. The avian with that extra buildspace can make forts, can go homes to fit more military, or can go really heavy banks to actually afford much higher wages, and likely higher GS/tgs/WTs or some combination of the 6.
And lastly, I'd pick avian because a perfectly played avian can kill an orc fairly easily by using its attack time. Introduce chaining in the mix, chain the orc's dspecs away, and ninja 20x its offense once its home is the only true way to prevent either side from 4-5x'ing eachother.
I like orc cleric much better than orc warrior. It allows the orc military to keep up with the avian military, it already starts stronger, and that allows it to keep hitting enemy support provinces much easier and often, which is really the key to the wars at those size, being able to hit the unbreakables.