The SWT 158s also have the same seats in standard and first as the 159s, the first class seats are very nice-probably the best on SWT.
http://www.flickr.com/photos/40172673@N03/8680283815/in/photostream
Not many TOCs have first class on 158s but I don't know of any better than SWT, although the old Northern Spirit first class was pretty good.
I travelled First in SWT on a 3 hour journey on a 159 last month. It was nice but to be honest I was left wondering if it was worth the money. Not because First wasn't good but because the standard and comfort of accomodation in Standard on an SWT 158/159 is so excellent itself.
If only every regional express DMU was as well appointed as those operated by South West Trains.
I'd genuinelly rather travel West Country to the Midlands and beyond on an SWT 159 than I would on a Voyager. They just seem like much better trains.
