To answer your question;
Compact is better suited for tight locations, is a new model and carries a price premium because of its small size.
Classic is a German based design which needs more space on the wall, but has been in production - with a few small changes - over the last 13yrs.
We have had a couple of isolated problems with the Classic heat exchanger but all have been dealt with the typical no fuss can do attitude that Worcester offer. Many Worcester installers have an 8yr warranty deal they can give you.
If you want to try a niche model from a manufacturer you may not be familiar with, that is recommended by vocal minority of enthusiastic technicians on here, go ahead.
thanks for,that Simon
But bear in mind there is virtually nothing to choose from in terms of efficiency between one boiler and another these days (due to legislation), so I'd suggest your choice should be down to reliability and support.
I can't be sure of either with new brands, so I'm sticking with known quantities. Worcester Bosch support is so good, and we do get the occasional fleece. But I'd fit them day in day out without the fleeces - which incidentally, most boiler manufacturers hand out.
Fact is, to find out whether the 'flavour of the month' boiler on here is any good, I would need to have fitted around 100 of them and seen them run for over 5yrs first. That's 100 customers I'm taking money off and using as test pilots.
Not going to happen.
If you search this forum going back over the years, you'll see Keston was recommended, then Remeha Avanta, etc. Both turned out to be a pile of plip plops. When Viessmann launched their cheap range in 2005 they were highly thought of here - 80% died in the first year.
If you must have one of the small volume niche imported boilers, get one of the specialist advocates on here to fit it, and let us know over the next few years how it goes. I think many (inc myself) would be interested and value the feedback.