We have been using PPB for around 3 years now. We are lucky in the fact we get all the equipment free including resin.
The resin we are given is around the £400/£500 per bag (25L) The Di vessels are around the 100L mark, so they take around 4 bags to fill.
We are experimenting with twin DI's but did run into some issues.
The resin in the first vessel lasted around 2.5 years running one van flat out and one van part time/full time
We currently use a resistance meter to check the condition of the water.
The vessels we currently use are
http://www.shop.elgaprocesswater.co.uk/c940-nuclear-grade-cylinder-5068.htmlI can't really comment on PPB VS PPM as we have only ever used PPB i suppose.
We still get spotting, had some complaints this year (less than 10)
At the moment we are experimenting with a RO system which produces around a 1000L an hour. It's essentially 5 40/40 machines but the filters/housings are longer.
Got no use for a 1000L an hour, but it something to play with when are bored at the site.
We get it all free as we provide an unparalleled service (different company, not window cleaning)