The Ingalls kit is a movable mount that replaces your upper arm mounts similar to the Progress kit for the 92-95...
The SPC kit replaces the ball joint between the "wishbone" and the upper control arm...
The Ingalls and the SPC kit will experience similar forces... they both replace supporting joints in the suspension system. Since the Ingalls kit replaces 2 joints, while the SPC kit replaces only one (both using one side as an example), common sense dictates that from the perspective of strength. This assumes that you may be introducing a weaker component into your system, so you want to play in the area that has less loading... assuming both areas are loaded equally.
Confused yet? Or are you following my jumbled line of thought? ...
Which area is more 'stressful'?
I think the ball joint (SPC) is more stressed than the upper arm pivot (Ingalls). The ball joint must translate the sudden and rapid vertical movement of the wheel into a circular motion, and it must help support the wishbone/hub. The upper arm pivots must just support the arm structure- the main forces in the area of the pivot are absorbed by the ball joint and the shock absorber. There's likely less shock experienced by the area that the Ingalls kit replaces, so it'll probably last longer than the SPC kit.
The Ingalls kit will probably experience less stress than the SPC kit.
Both the SPC and the Ingalls kit appear to be infinitely adjustable, from what I can see in the pictures. Unless there are notches or something that allows you adjust the Ingalls kit in discrete steps, I think it would be a pain in the ass to set up, and to adjust later. You have to make sure that both mounts on one side of the car are exactly the same- otherwise your upper arm will be mounted at an angle, and you'll get some funny caster and toe settings introduced to your system. On the other hand, if you want to force adjust your caster and toe with this, you probably could... although it wouldn't be a good idea. If the Ingalls kit is NOT infinitely adjustable, then it will probably be easier to use. With a setup like this, I would prefer to have a kit that only allowed you to have 4-5 settings- this way you can ensure that both mounts are set the same way. If once comes loose with an infinitely adjustable system, it's time to have some real fun...
On the other hand, the SPC kit only has one mount per side, so you don't have to worry about symmetry across two mounts at each wheel... just get the camber set the way you want it, then tighten it down. No matching required.
The SPC kit should be easier to adjust.
Of course, if the Ingalls allows you to adjust by finite increments, this doesn't matter.
The Ingalls kit 'pushes' the upper control arms towards the outer edge of the car, while the SPC kit only moves the top of the wishbone out. Some people have had issues with the upper control arm mount kits, in the form of the end of the arm getting hung up in the fender well under heavy compression. You won't have this problem unless you're correcting a LOT of camber, but I'd check your inner fender clearance just in case. My Progress kit only moved the arm about 1/2" out to correct -1.8 degrees of camber, so if the CRX's suspension geometry is similar, you shouldn't have a problem. If you're correcting something like -3 or -4 degrees, then you might have a problem. The SPC kit should have fewer clearance issues... but this is assuming that you'll have any problems like that at all.
Another kit you might want to look into is the Progress kit:
It's the same setup as the Ingalls kit, but it allows for discrete adjustment of the camber. No guessing required- just set it to correct a certain amount, then bolt it all in. I had one on my car before I nabbed a Skunk2 kit real cheap, and I was really happy with it.
One thing I would always be worried about with an infinitely adjustable camber kit is slippage... usually all that's holding your camber setting stable is friction between a bolt and a plate- and I think that's how the Ingalls and SPC kits work (from these pictures). Since the Progress kit isn't adjustable, you wouldn't have that problem. Check it out:
Well, there ya go... you asked for help- hopefully this doesn't muddle your brain too much. Keep in mind that this is all theoretic brain spew from yours truly, not info drawn from playing around with actual kits. I've never messed with an Ingalls or SPC camber kit- remember that.