(Well, this kind of turned into a wall of text)
Actually, soul power gives +40 for every rank. +20 static and +20 if you have a spirit alive, just like rank 1. You can test it.
Spirit ward vs soul power is interesting and usually match dependent. Spirits do more pure dps, but they have huge trouble chasing. Towers obliterate them, yet they are useful as fodder when killing a tower. Aoe destroys them quickly, but then you can get them back quickly too. Soul power is great for chasing, not quite as good when standing your ground. Don't forget rank 3 soul ward gives 200 armor and rank 4 gives 400 to everything allied. 8% or 16% more ehp from your base hp. Spirits can also be sent anywhere.
So, spirit ward is at its best when:
- Toe-to-toe conflicts or allied toe-to-toe conflicts where your spirits can be sent over
- Lots of autoattack
- 2v2s or higher are happening, dangerous to go in
- Fighting in open spaces without towers
Soul power is at its best when:
- Enemy is covered by towers
- Lots of aoe
- Enemy is fast / hits and runs / not in combat for long
The biggest factor I find is whether your enemy is relying on nearby tower cover or not. Towers are what stop your spirits in their tracks. Still, usually spirits are better unless you are steamrolling someone. Also it comes down to your point spending because spirit wards are not equal. Rank 3 ward is usually far better than a rank of soul power. The ward nets you +20 damage going past 6 spirits, you get more spirits, stronger spirits and 8% ehp for everyone. Soul power only offers +20 more damage than ward 3.
Ward 2, however is 2 spirits vs +40 damage. Except it isn't, not really. Ward 2 offers 2 wards instead of 1, extremely useful in many situations, and importantly you can actually use your spirits more. 3 spirits die easily and come back slower - using them will often lose you your +20 damage for having 3 alive. Hovering between 3-5 spirits you keep than bonus while at the same time using your spirits to a much further extent.
So it's like 3 different skills and you have 1-2 points usually. One point to use before lvl 10? Then it's ward 2 vs soul power 2. 2 points? Then you will likely get ward 3 early because it's awesome. But then maybe you're against a tb+reg+x team playing defensively and 3 ranks of soul power let you rip their faces off.
So many factors. You can even have the best of both worlds. You can go up both lines before level 10, playing a really focused dueling build. Usually happens in a 2v2 where it comes down to 1v1s a lot and you simply don't need surge of faith until later. It also usually happens more in a close game - when you're winning you usually need to pck up surge of faith 2 for pushing.
Also, penitence vs soul ward at level 1. It depends. Penitence 1 isn't for pecking someone. It's for when you've just gotten into melee and you put it between two autoattacks, not losing any damage, slowing them so you can be sure of nice follow-up hits. It's 4 autoattacks when ward+soul power bonus damage beats penitence 1 for damage. Better when laning straight away, but penitence is more of a team thing. Best example: xp flag in cataract. Both teams try and secure it and it's very important. You can't even get spirits here, but even if you could, penitence does more when 2 people are attacking the target. Both the 7% slow and 7% more damage make sure of this.
Is it worth delaying ward+soul power for 1 level? On cata I'd say yes, if you think you can win the middle. (especially since you'll hit 3 very fast after that). On other maps? Going straight to lane 1v1? Ward+soul power is better. Somewhere in between it's 50/50.