I do not have an ionics system, i had mine built by an engineer and having discussed with him the ins and outs of having a machine in a van, the reason you pay more for ionics is SAFETY.
The engineer who designed mine made a big thing about safety in a crash, and said that while you can try and design all you like, you only ever know for sure if you are actually in a crash.
I want a new system soon, and to be honest I am a LOT more bothered about safety now than I was the first time around. Having driven around for a good few years with a ton of machine behind me I have had plenty of occassions that have caused me to worry about my saftey in a crash. As ionics systems are crash tested i have to say I will be leaning more towards them in the future. Yes you pay more for that safety element but saving all the money in the world aint gonna do you much good if your are dead.
I seen the crash test videos on ionics website and when i done my bwca course, and no way will i be going anywhere near those systems that failed it and squashed the dummy, and correct me if i'm wrong but i think that was brodex.
I have seen how other wc have their systems fixed into their vans, especiallt diy jobbies. Not having my usual go at diy here but honestly they are just not safe at all. I seen one guy who had his 450litre tank wedged in with a bit of plywood. totally unsafe.
Ionics may well cost a few more quid every month because they are crash tested safe but it is a small price to pay for peace of mind IMO.