I know this question was asked 5 years ago and I hope the child of the original poster has grown into a solid ball player and is still tearing up the diamond.
But this is still a very relevant question and I'm sure many parents are continually looking for some guidance here.
I'd like to first start off by pointing out that I'm a little biased towards Pony League, but I'll try my best to be as neutral as possible in my comments.
In any district across this country you're going to find many different experiences at any given league. The whole "cut throat" mentality may exist more in the local LL of a certain city and the local Pony League is super laxed.
In one city there may be a very poorly run Pony League with no talented ball players, and a Little League that is very well organized and all the talent resides there. And of course it could be the exact opposite one city away. The latter is what I experienced during my years of play (1987-1997) before entering HS at age 14 in fall of 1997.
With that said, it's really up to the parent to scout both leagues in the area. There may even be Babe Ruth, Cal Ripken or other league types in the area.
If your child started in LL and you feel his talent (or even the other players talent in that league) isn't progressing at the rate it should, go scout the local Pony League and watch the players of the same age. There's no admission to go watch games at either league. After a couple innings you'll get a good idea if the talent is about the same or obviously lesser or greater.
Personally, I feel that the gradual progression of field sizes, and the kids only playing other kids within a 1 year age spread is astronomically better.
I played both leagues until I was 11... I vividly remember being 9 years old in LL and facing 12 year old pitching and it was beyond challenging to the point of intimidating and not fun. Some would argue facing tougher pitching makes you better. But some 12 year olds threw so hard I couldn't see it. It was hard to develop when I would only strike out or barely foul off some pitches.
But at 11 years old, I remember facing some 9 and 10 year olds and it was literally too easy. The pitching was pure BP and I hit an absurd amount of home runs that year (19 in a 24 game season) I did not go back for my final year at 12.
In Pony league, at 9 years old I would only face other 9 and 10 year olds.
In the region I played in (Tampa FL), the talent was much much higher in Pony League. I remember a friend of mine played only Little League for years and was a darn decent player in LL and even made Allstars at 12 years old. His first and only year in Pony League was age 12, and he did not get a single hit in PL... So, we literally had a LL Allstar who couldn't hit the ball in Pony.
The caliber of pitching was that dramatically different.
But again this fact may be different in another city, or maybe it is even true for my local league now that its been 18 years since I played Pony League in Tampa.
The drastic field changes in LL I feel don't fit the kids well either... At 9 the field is too big, and at 12 they are too small.
At 13 they are too big again.
This is vehemently apparent every year when watching the LL World Series. The 11-13 year olds are playing on a 225 fence... The home run counts are comical, and the distances many of the HR's clear the wall are just stupid. It's for this very reason that I find the LL world series painful to watch.
Comparatively, at 11-12 years old in PL, you'll be on a 225' fence with a 10' wall (similar) BUT, at 13 years old (Pony Level of Pony League 13-14 years old), you're on a 275' fence. Some parks are 260' in the corners and 300' in center.
There are actually a reasonable amount of singles, doubles and triples. Just watch the LL World Series and nearly every hit in the gap is cut too soon for the batter to get a double.
If LL transitioned to something between Bronco and Pony Level of Pony League for a field size, the HR count and distances the balls clear by would drop drastically.
My first year in Bronco (11 years old) I made the B all star team (as most 1st year guys did), and we played a Babe Ruth tournament.... Talk about an effortless domination... We literally 10 or 20 run mercy ruled every single team we played and took the championship game with no losses. We beat the runner up 14-4, and 22-0 in the final. Our team combined for 24 home runs. We felt embarrassed that we beat the other teams so bad
Again, these are only my personal experiences and therefore I can only recommend Pony League. But like anything worth doing, you need to do your research first and see how the leagues in your area are doing.
You may find both leagues are on par, and then your kid gets double play time and equal development with only minor rule changes.
But if either is drastically lesser, I would not recommend your child playing both.