And from a passenger point of view?
159s have nice big comfy seats in both First and Standard.
I've never been in a 158 where I haven't felt the operator hasn't been trying to punish me, whether it's for having legs that are too long, wanting to move my arms freely or not wanting to sit bolt upright for three hours. Oh, and in the case of the collapsed seat Haymarket horrors, inflict dull pain on my backside.
I'm sure that 158 aircon is more unreliable, so that's another form of torture.
It's a good question about the seats though. How can it be that 2+2 feels so comfortable on a 159 but so constricted on a 158? My theory is that you sink so well into a 159 base that you forget about the lack of space on either side...
Travelling between Bristol Temple Meads and Bath Spa, the oppurtunity is there to get either a GWR 158 or a SWR 159 for the 10/15 minute journey. I have been on both and if it is a GWR 158 with the original seats, there is little difference between that and the 158 and the 159 apart from the toilets, colour scheme and the 159 has First Class. It's different if the GWR 158 has Richmond seats, but I dont have a problem with them.
It's worth mentioning though, the 159/1s at SWR were converted from TPE 158s when the 185s were introduced. Incidentally, where all the GWR 158s with original seats came from, so it was just a refurbishment to make the same basic train different.
The GWR 158s with Richmond seats differ as these are mainly ex Wessex examples and the Richmond's replaced Chapman's and some other weird lower backed seats Wessex refurbished some of them with after completing the 143s, 150/2s and 153s with Chapmans.