im faced with the same dilemma myself... id like a very compact backpacking shotgun as well.
i have an 8-shot mossberg persuader with a 20" barrel and the thing is huge. even with a pistol grip its far too big to pack. the 8-shot mag is excessive as well - fully loaded the gun weighs far too much, and if youre not going to fully load it then the 8-shot mag is superfluous.
you can probably get an 18" barrel with a top folder to be more useful than a 14" or 12.5" barrel. with the 14" or 12.5" you wont be able to put a folding stock on it or it will probably be under the 660mm limit. you would have to use the legal loophole and put a pistol grip only on it, risk it being confiscated by a cop who doesnt know the law, and a short shotgun with a pistol grip is brutal to fire.
for myself ive decided on getting a top-folder with an 18" barrel. above 660mm so even cops who dont know the law cant argue, and once unfolded it becomes a fully functional shotgun as opposed to something with an 8" barrel and pistol grip that is useless for most applications.