@RCP2013 on here has installed both in his 4th Gen. Sumos in the front and Firestone Coil-Rite air bags in the rear. Maybe he can chime in to explain his rationale for going with bags in the rear vice Sumos. I know you mentioned Airlift brand bags in your post, but Coil-Rites are essentially the same product, minus the type of air coupler used, IIRC, so the Pros/Cons should be the same.

I have Firestone Coil-Rites installed inside the rear springs of my van. Install info here: https://www.siennachat.com/threads/traveler-the-family-hauler-roadtrip-van.72969/post-454676

The price seems to be roughly the same for each (Coil-Rites may be slightly more expensive) and both are 100% reversible modifications that achieve the same requirement to stiffen the suspension to prevent vehicle sag when the Sienna is carrying heavy loads.

My thoughts of the pros/cons:

	Firestone Coil-Rite Airbags	Sumo Springs
Pros	-Adjustable (5-35psi) for varying loads; you can dial them in to offer the desired level of support to maintain ride comfort. This is the only real pro I can think of that the Coil-Rites offer over the Sumos - but, IMO, it's a huge advantage.	-Simple; slide them inside the coils and you're done. -Can be installed in the front or rear springs. -When installed in the rear, they do not require bumpstop removal. -Other than inspecting them during tire rotations to ensure they're still seated properly, they're maintenance free. -I didn't see any mention in the installation material regarding their lifespan - I assume they'll last the life of the coil spring.
Cons	-Inherently more complex [install will take more time, greater likelihood for a system malfunction (leak, etc.)]; complexity will increase if you integrate a hard-mounted compressor / automated monitoring unit into the system. -Have a lifespan like any other rubber, or rubber-like, pneumatic component; from what I've read the bags generally last 4-5 years before requiring replacement. This maintenance cost will need to be factored in and will vary depending on how long you plan on owning your vehicle. There will be labor costs involved if you do not want to do this work yourself (probably 1-1.5 manhours). -Can only be installed in the rear springs. -Require rear bumpstop removal inside of each coil spring -Require some level of monitoring to ensure the proper PSI is maintained (no slow leaks); this requirement to monitor the bags can be negated with an automated system (there are a couple on the market). -You'll probably want to keep a compressor/pump on hand at least in your garage to control the inflation of the bags; something to add to your trunk as well for long trips - though this is generally a good idea anyway.	-Not adjustable. -Given your use case, the level of support offered by a Sumo Spring may not be enough to fully counter the additional weight/leverage of a heavy hitch mounted accessory (big bike rack, hitch cargo box, etc.); may not meet the full performance of a Coil-Rite air bag inflated to 35 PSI (max) pressure. -I have read that they can fatigue/compromise the non-Sumo'd coils in the rest of the coil spring; the thought being that if you remove the Sumo Springs, the coil's engineered ability to compress will be somewhat compromised as you've placed the non-Sumo'd coils under more stress than the Sumo'd coils (which would in theory fatigue them at a different rate) - again, this was just conjecture I read on another forum, I don't think anyone backed this up with a test (take with grain of salt).

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In summary, both products achieve the same basic result at generally the same price. The Firestone Coil-Rite bags offer adjustability (which is a big plus), whereas the Sumo Springs do not. The bags require more maintenance and recurring replacement costs if you plan on keeping the van over 5 years. The Sumos appear to be a 'fire and forget', maintenance free modification.

I suppose it comes down to the user and whether or not they value the adjustability enough to warrant the additional effort (and possibly cost) that comes with installing Firestone Coil-Rite airbags.

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*** if I missed anything, please feel free to comment and I'll add it to the above chart.

 

Thank you so much for going into all the details. I really appreciate all of your valuable input and the time you took out of your day to write this up!!