So, I'm going to answer this without prejudice, without any hidden agenda and with first hand experience, both as a consumer who has run both kits and also as a fully independent dealer who has supplied and fitted many of both the H&R T30 kit and the Solow kit.
It's important to remember that these kits are very different and are aimed at different customers, they are not competitors. The only time it becomes a grey area is if you want to run your van at a ride height in between the max drop of the H&R and the minimum drop of the solows, although there are other kits out there that fill that gap in the market, the merits of which can be debated endlessly depending on your agenda.
The H&R kit is advertised and approved for maximum 80mm drop. It can go lower and on a T30 you have two options at the front. The hub mod to drop the strut through the hub (not a fan of this personally, although plenty of people do it), or you can fit machined top mounts. Neither really affect the ride quality in a negative way, because they don't change the relationship between the strut and the spring. You can get about 12-15mm extra lowering with the machined top mount and up to about 20mm or so with a hub mod and a skinny CV boot.
At the rear you can fit an adjuster delete, which will gain about another 10-15mm drop, but this will affect ride quality slightly as it does affect the relationship between the shock and spring. It's not a huge difference, but it is a difference.
From everything I've fitted, run myself and test driven, I haven't found anything that rides better than the H&R T30 kit (when the rebound on the front is set correctly). The combination of build quality, ride quality and features at that price point is exceptional. I don't have any experience of the newest Bilstein kits that have popped up in a couple of places (not the B14 komfort, the really new stuff), but they would have to be something very special to beat the H&R kit.
Anyway, all of that said, there's no reason these days to run a kit lower than it is designed to go, there are just so many options out there to cater for every need.
Solows don't start at 100mm drop. It varies from van to van. I've had drops of 90mm at the cave at max height in both panel vans and Kombis, in T28, T30 and T32 but it varies depending on the weight of the van of course.
It's also not true that you can't run 20s with load rated tyres on Solows on T32, I've proven several times that it is possible.
Solows often require additional mods to the van to run them safely, so they need to fitted by someone who knows what they're doing as your taking the van well past the ride heights it was designed to see.
Any customer of mine considering Solows will have a full run down of the work that might be required and the pros and cons of running as low as the ride heights they offer. If people aren't 100% committed to them, or not sure if the 90-95-100mm minimum drop will be too low for them, I offer them an alternative.
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@Tyler846 question:
Yes, you can run the H&R T30 kit lower than 80mm.
You might choose not to.
To sum up other's comments/questions
Solow will start lower than that and go much lower than that, but might not be right for you. I love them, when they are correct for the application.
There is a kit out there that goes up to 90mm down that might be the right option when you think Solows are too low and other kits aren't low enough.
Hopefully that clarifies a few things for a few people.