First, I have to clarify the statement that "acoustic guitars have stiffer action" - there is a big difference between nylon and steel string guitars, and the setup itself can make a huge difference. And of course electric guitars (steel strings) have absurdly low and soft action, so they are the easiest to bar of all.
But in any case, the type, size and quality of guitar has been covered well here - although no-one mentioned the small guitars that are built specifically for childrens' hands - kids will distort their technique to cope with a full-size guitar, so take those videos that were mentioned with a grain of salt ;-)
But there is something obvious missing from the discussion - barring is inherently difficult for the human hand! First, it relies on the tiny muscles around the big knuckle in the hand - most of our hand strength relies on large muscles back in the forearm (you can see this by wiggling your fingers and watching your forearm). Second, it requires a very high degree of independence between the fingers, something that most people are not born with, and must develop gradually over time. The independence is required because the index finger must hold the bar firmly while the other fingers must move freely to take care of the rest of the chord.
There are lots of techniques for developing good barring and finger independence, and I keep students' expectations low as they gradually build up the requisite strength and independence. But there is one exercise I've found, in forty years of teaching, that makes an enormous difference - it was developed by a teacher of mine doing research at the San Francisco Conservatory, and it is simple and really works. Here we go:
- Hold your hands out straight in front of you Extend all fingers out
as far as they go
- Clench your fists
- Extend again, and clench - do this as fast as you can, getting full extension in the big knuckle each time.
Simple, eh? But after many fewer than fifty repetitions (unless you're unusual) you'll start to feel burning in your forearms. You can't injure yourself doing this - it's an isometric exercise - so "power through the burn" to a reasonable goal. After doing this twice a day, after several months you might be up to 200 repetitions, and feel a tremendous difference in your barring - good luck!