My only argument is that Warren Buffet probably knows best, and all of his money is in Berkshire. Past performance is an indicator of future performance, just not the only indicator. It's like saying that Tiger Woods might become a bad golfer. It's possible, but doubtful because of his past performance. Same with WB.
As for margin, it's not that complicated. It's a secured loan with 10% interest. At any given time I have a fraction of a share on margin (averaging 50% of a share, of course). Because Berkshire has such huge cash reserves, it would be virtually impossible for the stock price to hit 50% and thus trigger a margin call.
I will checkout those two firms you mentioned, but for now I'm happy with Berkshire.