I own a Blue Pig X – 17 inch – and had a go on a 17 inch Sherpa and a 16.5 inch Slackline.
Both felt felt a bit short for me – 184 cm / 6′ 1” – whereas the Pig never does.
I should be on an 18 inch really, so can’t hold that against the Slackline. 24 inch top tube seems about right for an 18 inch, 23 for the 16.5 sounds good.
Slackline is more of a premium product – better steel than the standard Blue Pig, also nasty paint on the Ragley nowadays. Not sure if they do the silly seat-post shim, but that has given me no end of aggro.
Whilst I have some affection for my ride, I wouldn’t get the modern version. Get an On-One 650b for less – more of less the same thing.
Dan Stanton was a diamond gent when I met him, the frame finish looks better. If it were a straight choice I would definitely go with the Slackline.