Note that you're asking about withholding, not about taxing. Withholding doesn't mean this is exactly the tax you'll pay: it means they're withholding a certain amount to make sure you pay taxes on it, but the tax bill at the end of the year is the same regardless of how you choose to do the withholding. Your tax bill may be higher or lower than the withholding amount.
As far as tax rate, that will be the same regardless - you're just moving the money from one place to the other.
The only difference would be that your tax is based on total shares under the plan - meaning that if you buy 1k shares, for example, at $10, so $1,500 discounted income, if you go the payroll route you get (say) $375 withheld. If you go the share route, you either get $375 worth of stock (so 38 shares) withheld (and then you would lose out on selling that stock, meaning you don't get quite as much out of it at the end) or you would ask them to actually buy rather more shares to make up for it, meaning you'd have a slightly higher total gain. That would involve a slightly higher tax at the end of it, of course.
Option 1: Buy and then sell $10000 worth, share-based withholding. Assuming 15% profit, and $10/share at both points, then buy/sell 1000 shares, $1500 in profit to take into account, 38 shares' worth (=$380) withheld. You put in $8500, you get back $9620, net $1120.
Option 2: Buy and then sell $13500 worth, share based withholding. Same assumptions. You make about $2000 in pre-tax profit, meaning you owe about $500 in tax withholding. Put in $11475, get back $13000, net $1525. Owe 35% more tax at the end of the year, but you have the full $1500 to spend on whatever you are doing with it.
Option 3: Buy and then sell $1000 worth, paycheck withholding. You get the full $10000-$8500 = $1500 up front, but your next paycheck is $375 lighter. Same taxes as Option 1 at the end of the year.
