CGB
Engines has as usual covered your query well.
If I could mention one more issue - dunno if you have come across the term Propulsive Efficiency? Essentially the lower the velocity of any propulsive air stream the less will be the losses of that system and so the greater the efficiency.
The Pegasus uses an exhaust velocity that is high compared to that produced by a typical propeller which is itself high compared to a large heli rotor.
So to utilise a given amount of horsepower in the form of a pure jet in order to give you thrust will be less good than using that same horsepower to drive a fan (as in the B).
The X32 used Harrier principles of pure jet thrust but managed to hover a weight some 8000lb less than the X35 using a fan and the same basic engine. This was just down to the propulsive efficiency effect.
Of course in the case of the B the greater propulsive efficiency is only achieved at the expense of much complexity and potential unreliability, cost and weight . However it was still judged by our betters to be the solution of choice.
I am afraid I am not a big fan of the B for the USMC because I think a spec that calls for supersonics and stealth in the CAS role is wrong , especially if you throw in austere sites. But that is a whole different topic.
I completely agree that should a customer insist on having a supersonic vertical lander then the B has the best configuration to achieve this that we know of today. If the guys currently working on anti-gravity paint get a breakthough then that may of course change.