I echo
@Rarity's comments. We know that IB explicitly pays 50% of interest charged (it's in their program documents) to the lender of the shares (minus a really small sliver to pay for transaction costs, ~1% or less). Fidelity sets the two rates separately, so at times, Fidelity will be keeping >50% and will sometimes be keeping <50%. Somebody posted a history of the lending and borrowing rates from Fidelity tracked over a few months - over that period, it looked like Fidelity was paying about 55% of the lending rate to borrowers.
Over time, I figure everybody will be in the vicinity of 50/50, and will maintain similar lending rates. Market pressures will keep them adjusting back towards each other.
I personally went with Fidelity as they seem more friendly for infrequent traders like myself. IB wants a large account balance or frequent monthly trading ($10 in commissions / month), while I don't typically need all the fingers on a hand to count annual trades. And I hate paying fees, so being able to have my brokerage account with no monthly charges is important to me
For lending your shares out, I consider understanding the safeguards that protect your shares to be paramount. You're protecting from a number of counter party risks. They all sound scary, but you've also got some powerful interests working in your favor. The most important, IMHO, is the power of your counterparty (your brokerage) to sell anything and everything in the borrower's account to reacquire your shares to return them to you. The collateral backstop is primarily to protect you from your brokerage being unable to perform, not so much the borrower's ability to perform (though I expect your brokerage gets the cash collateral from the borrower).
And you still risk selling your shares today, at yesterday's closing price. In a period of sustained stability in the share price, that isn't much of a risk. Where this is important is if TSLA moves 50% in a single day - that'll create large demands on short sellers to supply additional cash or close positions, and in the extreme of your brokerage being unable to perform, result in you receiving cash back for your shares instead of the shares (that just went up 50%). Of course, I think that looks like bankruptcy for the brokerage (they didn't return your shares, or update the collateral held by the 4th party at the end of the day), and they are understandably loathe to allow that to happen.
The financial strength and willingness of your brokerage to keep you whole when they borrow your shares is part of what you're choosing, when you choose a brokerage to lend your shares to.
