I took HEL-TXL//DUS-HEL with AB last month. HEL-TXL was maybe two thirds full, DUS-HEL fully packed. I must say I did not expect anything, and was quite pleasantly surprised; only the LTU plane from DUS seemed a bit worn out, but it might have just been the colour scheme that I didnät like.
However, if the price difference were only small, I'd definitely prefer AY, even with the new Recaro seats (which I don't find all that bad in fact). However, Finnair to/from TXL seemed to be almost zeroed out for all suitable dates.