The Stock headgasket are at their max at around 18psi.
The stock cast piston will melt as they cannot take this sort of heat (25psi) for too long periods and you don't want to be making mistakes while tuning because at this boost level many standard things can brake, stock valve springs aren't strong enough the standard cam's don't have high enough lift to let 25psi of air out in and out of the engine.
At 25psi you will be making a fair decent amount of power, the stock con-rods also wont handel the pressure & load on boost.
The RB25 is a strong motor for what it is but once you turn the boost up things start to go wrong.
Robo,
A bigger turbo will cause more dammage as it can physicaly flow a greater mass of air than what a smaller sized one would, eg: If you were running 18 psi through both turbo's and you got a boost spike of 23psi the smaller turbo has more chance of running out of its efficency range their for won't make much more power but a bigger turbo will keep going.
To answer you'r question, i'd would say boost, because with out boost you don't got no power but it also comes down to tuning, alot of the time this is what kills engines.
I'm sure SK will let you know what the RB25 is capable of.