Not sure barrel length would have much impact on accuracy as there are plenty of pistols with much shorter barrels than these 2 that are extremely accurate. If it was me I would ask myself which rifle would I keep if I could only have one and convert the other one!.Brian
The Air Arms Pro Sport barrel is I believe ~ 9" as is the Air Arms TX200, both rifles are very successful FT competitors. They are both deadly accurate rifles and are tuned to 12FPE on a regular basis. If it were my project the the D34 Meisterschutze would be my first choice. In fact if I weren't so happy with my D460 I might be tempted to install a low power kit in it.
I think you're going to like the change. Variety makes things more fun anyway. I'd use the short barreled rifle because I read in one of B.B.Pelletier's blogs that a springer needs only about nine inches of barrel, because of the small volume of air used to propel the pellet. Plus the nice balance that you seem to like in the carbine also makes it a good choice. You didn't rise to Expert status by not knowing stuff about air guns, so go have fun and tell us what you think!
You may wish to check Connecticut Custom Airguns (Hector Medina) for a suggestion on a spring or kit to fill the bill. He shoots Diana competitively and has a LOT of answers!! As far as which rifle - I too would probably make the Meisterschutze my project gun. This process is reversible and I do love the look of my Meisterschutze. If it didn't already have a Maccari kit in it I might consider detuning it.
Shorter barrel and a WOOD stock ... for a 12 fpe FT gun would be my suggestion.